No.. I never "railed against" Mythic+ style dungeons. I am IN FAVOUR of them; i'm just not foolish enough to think that they are a perfect solution to all the woes, but accept that nothing will solve all the problems.
Telling you in clear terms why your solution is no solution at all is not "railing against unpredictable opponents"; Try reading sometime. It really helps with a text based forum.
You make the existing mechanics of the fight more punishing (damage, absorption of damage, need for interrupts on mechanics), then you add more mechanics to the fight at certain difficulties that require the party to not just throw themselves at the boss but pay attention to add mechanics or other things to take direct attention off the boss.
So what you want is a virtual intelligence? Dear god... I'm sorry, but you're not going to get that here.. ever. So it's not much of a solution, is it?
Considering the fact that you're suggesting creating a virtual AI for boss encounters, and is not something that can be adequately created by any AAA developer at the moment.. yes, the attempt will kill the game just on development alone.
Your solution is impossible for ANY GAME, not just MMOs with PvE... Name me a single game where the enemies don't act on prescribed processes and instead "think, observe, and adapt" to their opponents.
Encounters are designed to create an atmosphere for the fight. When they design the fight, the developer team have to think "what do we want to do with this fight" and try to give the audience something that will challenge them but not lock them out of chances of victory. If it comes closer to the latter than the former, than they are able to tweek an encounter so more can have a chance at beating it.
Do i always agree? No; but "let's make a virtual AI" isn't the answer to this bloody question.
And you've climbed out of the box, down the hill and into the rabbithole labled "wonderland" and are having tea with the queen of hearts.