Usefulness of high AC at high levels (Tier 3 and 4)

Hi all,

I'm building a character at level 10 who will play through to level 20. I've heard that AC-focused builds become less effective at higher levels, but, looking at the Attack bonuses in the MM, it's not clear why that is. For instance, an ancient red dragon has a +17 attack. It seems like going from an AC of 18 (plate) to an AC of 25 (plate+shield+shield spell) would take one from being hit 95% of the time to 65% of the time, which adds up to a lot fewer hits. What am I missing? Is the idea that distinctions between ACs in the 13-18 range matter less at high levels, so acquiring heavy armor on its own is less of a big deal?

High level AC builds have three problems:

1. it's hard to raise your AC as fast as monster attack bonus increases; reducing hit chance from 40% to 30% is a 33% increase in toughness, reducing hit chance from 70% to 60% is a 17% increase.
2. Damage that doesn't go vs AC is increasingly common.
3. Your damage suffers significantly, because there are no damage-boosting feats for single weapon styles. Being hard to hit doesn't matter if the monsters just ignore you while beating on your less armored allies.

My recommendation: if you want to play an AC focused build, play a paladin -- they get a lot of their damage output from smites so the lower base damage of one handed weapons matters less, and they have aura of protection to deal with damage that doesn't go vs AC.