Some games already have adaptive AI. Fight Night had adaptive AI and that was 2 gens ago.
The real reason gameplay has capped out is because devs have to destroy and rebuild so often. If they were able to keep compiling data instead of needing to scrap engines constantly then features that were already introduced wouldn’t be lost in transition. Sports games are a perfect example, they constantly go through the same cycle of being barebones when they go to next gen and slowly get better, then start over. As opposed to carrying over the features that were already there, features are taken away and replaced.