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

Saying high AC is less effect at higher levels is disingenuous. A high AC is always better. The higher the better.  +19 is the highest (30 CR) to hit modifier available without a magical plus weapon. You are more than likely never going to face 27 to 30 CR monsters and if you do, it wouldn't be frequently. 

Most campaigns will typically end facing 12-17 CR monsters and high AC is definitely favorable. In your dragon example, it would be the melee folks leading the charge against such fiends, while the others stand back to cast spells and the Rogue sneaks around for advantages on his attacks. 

ACmaxxing is generally not good
the DM cannot allow a matimatically unkillable character, where they don't get hit when compared to everyone else,
you need equally good saves or the DM will just spam saving throws
and if you are missing some common save, the DM will simply throw that save at you so you suffer equally as everyone else despite you investing in being unkillable
and, by investing so much into being unkillable in both AC and saving throws, you lose actual ability to affect the battlefield
however,
i did once optimized a abjuration wizard with one level in artificer that tried to do just this,
it had a lot of AC, wore med armor and held a shield, casted the shield spell, took a lot of planning, attuned items like staff of power, cloak of displacement, ring of protection, until AC and martial attacks basically always missed, and spell saving throws were disadvantaged cuz spell resistance, and resilience(con),
and it still sucked at STR saves and often messed up DEX saves,
it only kinda worked cuz i was still a wizard and therefore had all wizard spells, which helped a lot, i just played control wizard that happened to be really hard to hit when you are in their face
but it felt in normal gameplay as a slightly worse wizard,
TDLR
even when ACmaxxing kinda works, it still does not feel that good