We're talking about a linear scale ... core D&D basic concept.
You're assuming I think the mechanics of the "diminishing returns" AC are good too and accept that.
They aren't. In that case, I think you'll find I and many others would rather see the range of mob to hit increased to a wider amount, thus making a wider range of AC useful.
I'm sorry you can't see my perspective. Making things actually "harder" numerically on Elite Epic 25 than 15 is FINE.
I don't care if the caster who's geared for epic 25 can own the devils in VoD normal. I do not see a problem with that at all.
I'm a proponent of using or extending existing mechanics. Plenty of spells offer bonuses / penalties based on number of targets in the area to be affected; that would work fine and prohibit mass wails. If only a few spells are a problem, MODIFY THOSE.
I get it, we all like to invent new things. Sometimes you need completely new things. Sometimes though, you may be ignoring simpler answers.