Have you had a successful T3-4 ongoing campaign?  All our ongoing games have ended shortly after we got to L13.  We have run some high level one shots that were an absolute blast but it felt really hard to sustain.

I know this is a common issue so I'm curious how/whether others have solved it?

This is a common issue - Wizards has spoken multiple times about how most campaigns tend to fizzle out for one reason or another around level 10.  In one of their recent videos, Wizards acknowledged they have contributed to this problem somewhat - because their data shows most campaigns fizzle out in the low or medium levels, they focused their products on those levels. This, in turn, left higher levels with less content, making players less likely to play at those levels, creating a bit of a spiralling situation.  Wizards has said they aim to work on this—and I expect high level content injections is one of their main reasons behind books like Fizban’s and Bigby’s.

Personally, I design campaigns to reach level 20 and have a couple successes under my belt.  The secret I have found—be very careful with pacing. These are campaigns that can easily last longer than a year in real-world time.  You need to walk a fine line between revealing enough of the story to keep them interested, but not so much there is no mystery—no reason to keep going.  Striking that balance is difficult and requires patience and the ability to rapidly shift elements of your campaign (without your players knowing your plans have changed) to keep up with the evolving expectations and interests of your players.

Out of the last seven campaigns I have run in the last 10 years, only 2 didn't go into Tier 4 (they capped at tier 3). The last campaign was three years long, and the zero session for it included an acknowledgment that sacrifice would be needed during the end of the campaign (forgotten, of course, but I earned it and it was really emotional for everyone).

In other words, I frequently run high level campaigns.

That hasn't always been the case. Early games were mostly capping out in the 7 to 9 range, but also we didn't do campaigns, we did simple dungeon crawls and odd asides and it was the 80's and we were young and were not settled into our adult lives and all that.

Commitment can be a big challenge in these larger campaigns -- the folks need to not only be interested in the campaign as a whole, they have to be invested in their characters way beyond some kind of mechanical advantage to them. The single biggest cause was the dropping out of players in a game -- and usually due to 'real life".

THe next thing was that it really wasn't a campaign. It was a collection of adventures that weren't themselves connected by a story -- it was far too episodic. THen we have the issue of the storyline was boring, or the players weren't invested in it. That last bit was why we switched to the "sandbox" or "open world" style of adventure for most stuff -- go out and explore the world.

That led to a lot of humor when isekai anime became popular, lol. The kind of boring "here's a fetch quest", "here's a kill quest", "here's a run quest" model was so firmly entrenched that we laughed because that's like 70% of the early adventures. But they always had hints and threads even if they weren't tugged on, they were known and seen and they would add up to something, even if the sum wasn't known.

That's why there is so much emphasis on story in high level games -- to get to them needs to build a character, to know a character. Pregens just don't have that kind of oomph to them, so while they can be fun to do as a like a on shot, they have a lot of strange uses to them.

High level campaigns are played entirely differently from lower level campaigns. They require that players be invested in the world around them, in the peoples and places and things that have nothing to do with any adventure. This is why downtime stuff is important. That they have friendships and memories with NPCs, that they have some personal reason drawn from their back stories and their backgrounds, the history of olay they have. 

They also require something more complex than a final boss fight. At Tier 3 and 4, whatever the characters do should have an impact on the larger world -- it should change something significant that can be seen. The way that we explain it to the younger folks in our group is very close to this

By the time that Frodo and Aragorn are standing in and before the lands of Mordor, they are level 20 characters. Level 5 and 10 and 15 do not face the God of Evil, they face the simplest of his minions. By the time the Nazgul faced Frodo, he was level 15. When Merry and Pippin rode Treebeard to the drowning of Saruman's Folly, they were 10th level.

It drives home the scale, and also reinforces the way that one has to construct a campaign in order to run a big one. The BBEG that some 7th level fighter may celebrate beating in some lair was still the smallest of the small pond scum in the grand scheme of things, and yet when you run it it has to seem like the important victory it is, the moment of proviging that it is.

WHich is why a lot of folks have a hard time if the modules only work so much. A good old fashioned dungeon crawl can be great, but and some groups will never get enough of them, but they tend to not be as common in high level games because the stakes aren't something that needs the kinds of features and special abilities that exist then.

Here's an example: In a given world, Goblins are considered by everyone to be the bad guys. "No such thing as a good goblin." the folks say. IN a standard set up, Goblins would not be a playable PC race. 

However, in a tier 4 game, one of the outcomes could be that the PCs make people see that not all goblins are evil, and so actually make being able to play a goblin is possible.

Insert whatever race you want there.

Another thing is that high level campaigns mean you need more history -- and one cool thing to do there is to use older characters and adventures as "legends" or stories told around the new characters. Exaggerated, of course, or always off a little bit, because these are all fifth and sixth hand retellings, lol.

In short: you have to make the players feel like they are part of the world to do a good Tier 3 or 4 campaign. THey have to be part of the story, but not trapped into a fixed set of things they must do.

They have to have subplots for them. They have to be involved in the creation of it, they have to feel like it matters *emotionally*. We put a lot more work than even Xanathar tosses in to backgrounds and backstories and personality during character creation -- and we revisit it a few times (3rd, 5th, 9th, and 13th levels specifically). To really build up these stories and get the characters involved.

I do massive complicated outlines that have a lot of connections from different tiers, and when they do the character creation and we create their backstories, I add those things into the whole. Spread out, with any resolution coming late in tier 3 or early in tier 4 and the resolution should at least point them towards the BBEG.

In none of it is there any kind of "the players will do this" -- because the players probably won't do it, lol, but also, remember this is a sandbox. I need to give them reasons to do something, not force them into doing it. And I fail far more than I succeed, but I still get there (in no small part because everyone knows at the start that these are the goals of this game).

Interestingly, I've been playing off and on since the early 80's, and it wasn't until 5e that I actually had a level 1-20 full campaign run to completion. (There was a 3e that started at 15 and run into Epic levels that we at least completed the entire story.) So it is NOT easy. Schedules was nearly always what did us in.

But AEDorsay has some great advice - very high level campaigns are fundamentally different than lower level ones. For one thing, the power levels being thrown around are such that it most of the time I don't even expect the opposing side to stand a chance against the PCs, but we run high fantasy style campaigns, so the players really enjoy being able to tear through their enemies as opposed to groups that need to feel the fear that they might lose every battle. I can still put fear into them sometimes, but it's a fine line.

Typically, I put the fear into them instead with - their hometown is going to be invaded and obliterated if they can't stop it. The souls of their fallen friends & neighbors will be consumed and destroyed forever unless they can stop their enemy soon rather than later. As well as things like setting up an enemy to be beyond even challenging (either god-like powers, or even no idea where or how to face) unless the PCs do at least some of A, B, and C, and then it's at least a possible fight (and depending on how many of A, B, and C they complete and how long it takes, that battle changes). There's still little doubt the PCs will win, but usually the threat isn't to the PCs, as well as, half of the battle is won ahead of time with side quests.

Lastly, like AEDorsay, even though I'm playing in the Forgotten Realms, I let the PCs have major changes to the setting. Last campaign started as Out of the Abyss, but wound up with the PCs overthrowing the Baenre family in Menzoberranzan creating a new (and still unstable) political order and created a shaky alliance of Underdark settlements that the PCs and a new generation of NPCs following the legendary PCs are working to maintain. Current campaign had destroying Szass Tam forever and kicking off a Thayan Civil War as well as re-awakening an ancient Netherese flying city that is currently floating over the Dalelands. Some might flip out at the thought of PCs overthrowing Menzoberranzan's ruling family or killing Szass Tam and always figure there's some secret plan those enemies have to be brought back and return everything to status quo, but in my opinion, that is a terrible idea! Even a setting like Forgotten Realms is just clay to build your own stories with, not some set of restrictions that you aren't allowed to alter. Heck, if a group of high level PCs can't leave some impact on a campaign setting that is likely to make parts of future D&D products incompatible, then what's the point? ;)

Quote from Caerwyn_Glyndwr >>

The secret I have found—be very careful with pacing.

 I would agree that this is key. It's not easy to make a coherent story arc that goes on for 20 levels without just sidetracking into "filler" to pad things out. Pacing is a tough concept that even a lot of popular professional media doesn't always get right. And they don't have to worry about players throwing wrenches into your plans.

My current campaign is at 13 and could be resolved in another level or so, depending on the party's choices. But I do have a quite a bit more planned out if they decide to do the hard thing.

Heres the thing...
why are campaign fizzling before getting to tier 3 and 4 ?
the simplest answer is time constraint !
explanation time !

each players have a life to manage, most of them do not know where they will be in 5 years, let alone 3 years, let alone 1 year. for a campaign to reach tier 3 and 4, one has to dedicate about 1 or 2 years of his time to a single character and campaign. that is a lot of comitment and none knows the future, their job might breaks their time, they might lose their job, they might form a familly and have less time. all of this factors into play. you cannot expect a campaing to be run entirely if you play only 1 time per month and then after 4 sessions pass 6 months before playing again because everyone doesn't fit up in schedule...

my campaigns all go for that level 17-20 ish final boss as characters gets to fight for the gods themselves.
but out of 30 years of DMing, its only recently that i could actually finish 1 campaign to its end without losing my players to life itself. i was blessed recently as i concluded 3 campaigns entirely with 2 different groups over the last 7-8 years. but reality is, they aren'T high schooler anymore, some bought a house and have to work hard now. one has children and cannot really be with us now. 3 others have moved away to another city in order tolive their dream which, by the way wasn'T to play D&D forever... so after 8 years i'm stuck trying to find new players to replace them and both my groups have turned down from 6 players each to a mere 3 players, 2 of which are the same people playing in both games.

the real reason games dont last is not for lack of content. its true WotC has a small part of it... but the real culprit is always life !

heres a couple way of making sure your games can last longer...
- never wait for everyone to be there, if you have at least half of your players at the table, then you should play, the others are just AFK. if you wait for everyone, the more people you try to get together the less chances you get at actually playing. so just make them AFK and play still if you have half of your players.

- make sure people you play with knows the commitment you are asking them. most people think D&D is a once in a while thing, they dont think its a weekly thing. they think its like any board games or video game they can just chill and play when they feel like it. make sure they know you want commitment to the campaign if you want it to go far

- life is life, people come and go, do not hesistate to have more then 5 players at your table. even more if many players have scheduling conflicts. that way you always have a full group reguardless of the schedules. as an exemple, i had up to 12 players at one point in the first campaign, but at all times only ever have had 6 at the table. game was always progressing every week. but you have to ready that one day where all 12 players will be there though... that happens once ina full moon but man was that night fun as hell !

- life being life... you also have to contend with the fact that your campaign can die at any time due to players moving on. the only thing you can do about this is to have characters premature ending, exits just in case. as an exemple i had a player who wasn'T sure he could go the whole way. at some point i ended his last session at his ranch saving his familly, his chance at a sunset. the next week he announces me the good news that he can go on till the end. we start back from his sunset and he says, i'm not done guys, i'll fight that big guy with you, you helped save my familly, i'll help you fight off gods. and we had a blast for another year. but every characters had a premature ending setup just in casethe inevitable hapenned.

those are the only tip i can give to aspiring DMs who hope for a long campaign.