Aye, that makes sense. You've got a cabal of wizards supporting the hero facing the demon, he should be buffed pretty good.
Rogues with 18 dex at level 20? I'm sorry, but even if you're not even trying, you can get 20 by 20. That should be the target. End game monsters should only be beatable with people who take their training (in-game and out of game) seriously. The system mastery level shouldn't be too difficult.
I'm fine with balancing the game around having +1 weapons but 20 in their attack stat at level 20, don't forget rogues will be attacking with their dex and not their strength, and if they are, they deserve to fail!!! None of this "protect players" from mechanically bad choices nonsense. It's not that difficult to know you should pump your attack stat, nor expect even n00b players to understand that, even if they don't min-max they should at level have 18 by level 20. But don't balance the high end monster AC around that, balance it against the 20 stat because with bumps and point buy, really, everyone should have it who doesn't have a very good reason to not have it. Or if they don't, that's too bad, they'll just hit a little less often, no big deal either.
The more I think about it, the more I agree with those who said 2d6 + 4 should be the default recommended die rolling method, after the current 15 max-per-stat point buy. The adjust the enemies AC up or down.
I don't want a game where casual joe blow is expected to win against a huge red dragon, because the "math fairy made it so". You should have to have lots of luck, planning, buffs, and very good training, practice, and planning. If you don't, let the characters die. A game where it's too easy to kill of end bosses (and balancing them vs casual gamers is a sure way to do that), is a huge FAIL in my book. Keep the tough monsters, tough, and account for EVERY bonus that's in the game to be used and stacked, where possible, when doing the AC and HP adjustments.