Quote from HamFrozenSolid >>

• It could be used as an alternative to Feather Fall, but the falling creature would have to fail the save for it to work. A target cannot willingly fail a save unless the spell or effect says they can, such as the push/pull on the Telekinetic feat.

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 In the 2024 PHB, creatures will be able to willingly fail saving throw according to J. Crawford and latest playtest 7.

this could also be used to immobilize a beholder

Would the affected still go downwards if triggering a pit or slide trap?

And what is the DC of that Strength Save?

It doesn't NEED to have any more saves, the dragon just needs to use it's breath weapon on you and break your concentration. Also, assuming it's max range, the would take a target 5 turns to get down, and it's not gonna even take damage from that. Heck, as a dm, i'd even argue that it can maneuver itself, as the spell specifies it 'safely descends' ie, there's no forced movement, so i'd, personally, rule that it could glide around. I cannot think up a single situation were this spell would be better than hasting hold monster, or just taking pot shots at the dragon. And it's just way too risky for a spell caster to do. Imagine this. Dragon is flying away, or preparing for an attack. Its in the sky none the less. Assuming a party of 5 with 3 casters, lets say Sorcerer, Warlock and Wizard, all with earthbind. All of them casts spells to get rid of the dragons resistance (dragon has to be in range for those spells, so lets say around 90 ft, 120 if we want to be generous, though i don't think there are too many spells that force saving throws and would be useful in a dragon fight with such a long range, but lets assume 120 ft). 
Now, assuming the dragon has abysmal luck, and fails all three saving through, lets say the wizard, lets say a divination wizard specifically to ensure it fails this save 100%, casts earthbind, before its next turn somehow, in order to get that thing down. Dragon rolls, wizard uses his ability, fails the save, dragon starts descending. On its turn, the dragon is gonna move downward 60ft. Now, there are two situations here that could happen. 

1) The wizard has used up his movement speed this turn, which is really really bad. Dragon descends 60 ft, looks at the person who casted the spell and took away its flying speed, and decides, screw it, i got a 60 ft breath attack, might as well use it. Blasts the party, even if the caster rolls well, that's a dc 21 if we assume an adult. Granted, if it were a young one, it's range is only 30 ft, so it's not as bad, but if the dragon can glide, or, if it was in range, that could still be an issue. But on average, that'd be something like, 50 something odd damage, 25 halved and rounded down, cause i cant go find the sheets at the moment for exact numbers. So, wizard's concentration very likely ends, and if it doesn't we move over to scenario 2.

2)The party has two turns were the dragon can't quite touch them. The wizard run away 30 ft, then another 30 on his turn to turn and take pot shots at the dragon. now the moment the dragon is on the ground, who's he gonna go and pounce on directly? The guy who earthbound him. Even if it takes him two turns to get to him, even if he succeds against the breath weapon the dragon will without question focus on him, the wizard will soon enough have to deal with 3 attacks minimum, dealing 2d10+ something, and i think a d6 too? (assuming a young dragon here, literally the BEST case scenario). If he's still alive, if the dragon does not kill him cause, well this guy can prevent it from escaping, his concentration is broken 1000%.

I guess there is the scenario three, were the wizard uses those 2 turns to get 90 ft away, 30 and a dash, but that a, splits the party, and makes it a huge pain for a battle map, b, the martial characters need to run around to get to the dragon, while spell casters are unloading. So, only 2 our of 5 players really get to do anything, while everyone else is trying to catch or run away from the dragon. Which if you're not one of the two players, is not very fun. 

Now notice how many assumings we have here? How many things need to go the parties way, from shitty rolls on the dragon's side, to exactly the subclasses needing, positioning being juuust right, and, most importantly of all, assuming they KNOW the dang spell. 

For wizards and druids, it's not the worst, they could arguably prepare it, if they somehow knew they would be fighting a dragon. But for sorcerer's and warlocks who pick spells and, except from very specific situations, will have to live with these spells for the rest of their lives? Why the heck would they choose earthbind of all things, instead of different, far less situational spell, to take with them? Warlock's especially with their limited resources. Unless it's part of an extended spell list, which i dont believe it is, there is no reason to get this spell at all. 

All in all, this is a very situational, very niche spell, that requires a lot of things to go its way for it to be useful at all, and even then, it's effects can be easily countered by breaking the caster's concentration. 

Also, i'd like to note that, a) while this could be used as a feather fall spell, in a way, the dm needs to allow for someone to choose to fail a spell, otherwise it's not guaranteed, and b) while you, technically, could target a creature that's flying on a machine, like say, an airship pirate smirking down at you from the edge of the ship as it takes of, there is no forced movement, so the ship would fly out of range, and his movement speed is not impaired in any way, so, once the ship flies off range good job, you wasted a spell slot, and you need to target a creature, so, the airship is not an option. 

To make a long story short, this is very far from the an excellent spell, especially for the purposes of grounding a dragon, needs a lot of things to go its way to work properly, 2 out of the 4 classes that can use it have little to no incentive to take it, due to how situational it is, while the other 2 need to know in advance so they can prepare it. All in all, bad spell 3/10, and it gets those only for its potential to be a semi good feather fall with a long range.

Could be good for a campaign where you know you'll be fighting lots of flying creatures though, but that's about it.  

Can you use this on water to potentially drown a creature?

Although I find your assumptions interesting and quite detailed, I believe they are making your analysis more compliacted than it should be.
I'd simplify that in two broad scenarios since all considerations about attacking the caster /split of the party / movements/ etc applies to every (dragon) fight, regardless of the used spells.

Additiona note: GLIDE during descent: that would be DM ruling. It is not writtend anywhere that dragons can glide if affected by this spell (and I'd rule that 60ft/round will also eat-up monster movement).

Scenario 1 - dragon (or other monster) is flying

• "Earthbind" has 300ft range - that is a HUGE distance
  
  • Casters can use "standard" spells if dragon is within range and earthbind for bringing it down if too far away
  • Also, earthbind is just 2nd level - may be a very good way to force dragon to consume its 3/day legendary resistance
  • Furhtermore, martial characters without flying ability have NO WAY of effectivery fighting a monster at long distance
• spells & ranged weapons can target monster multiple times during the few round of descent (depending on altitude)
  
  • Once on the ground, the martial characters will soon be in meele protecting spellcasters
    • BONUS - dragon cannot fly away from the fight (not even through legendary wing attack)

Scenario 2 - dragon (or other monster) is on the ground

• Melee characters and spellcasters can go nova on the dragon.
• "Earthbind" is still advantageous in two situations:
  • preventing tactical flying movement during combat --> dragon need to move on the ground = difficult terrain due to party fighters + AoO risk
  • preventing "flying" escape from the fight: 300ft is a huge distance and dragon will be forced to try escaping "on foot"

Comparing vs hold person:

• same level
• PROs: HP has more crippling effect & can be upcast
• CONs: only works on humanoid & subject can roll save every round

Comparing vs hold monsters:

• CONS: 5th level vs 2nd level
• CONs: only works on humanoid & subject can roll save every round
• PROs: HM has more crippling effect & can be upcast

The way I interpret this spell is that the target falls the 60 feet per round on their turns. The question is, does that use their movement for their turn, or can they still move parallel to the ground (or descend).

Quote from TheNorsePantheon >>

The way I interpret this spell is that the target falls the 60 feet per round on their turns. The question is, does that use their movement for their turn, or can they still move parallel to the ground (or descend).

I think that since it doesn't say that it uses their movement, it doesn't. Generally, when a spell or other effect forces someone to use their movement in ways they might not want to, the spell explicitly says that.

 I would interpret "airborne" as "levitating" - as those creatures are usually fine with zero flight speed, the spell will bind to Earth even those.

so i had came across a situation. when your earthbind could you still teleport?

Was thinking about this spell and think it's a great spell for Eldritch knights or other spell casting martial who do not use a lot of range. 

Edit: and for spell casters, sometimes the best buff/cc you can provide is letting the barbarian/paladin/fighter finally get their hands on the manticores for a change

Edit 2: agree to the should be a lil more powerful or upcastable (or able to switch targets if yours recently met a tragic end by barbarian who entered a rage of joy upon finally being able to kill the damn flying thing)

What happens if you cast this on someone that is swimming?

They can no longer fly. 

I think this meant to be used on creatures that only have flying speed like a beholder.

lure some big flying thing out above water, cast this, and whatch it drown. i am looking at pretty much everything without amphibious and with a fly speed. (most ancient dragons, angels, demiliches etc.)