They already do that. There are plenty of seasonal mechanics that made it into the game permanently. Infernal hordes, horadric strongrooms, Belial ect. The issue with mechanics involving itemization becoming permanent, is that it limits the future possibilities of new seasonal mechanics, and in the case of chaos armor, it also completely obsoletes legendary armor and aspects. We only have a set ammount of item slots and gem slots, if any seasonal mechanic involving either becomes permanent, that slot is forever locked in for that specific mechanic. What then, when all the slots are used up? No more itemization mechanics in seasons?
You mentioned D3 as a example, well if you think about it, D3 steered clear of making any seasonal itemization mechanic permanent, for the very same reason.
Personally, i would rather see chaos rifts become a permanent addition, instead of chaos armor. Chaos rifts really spiced up NM dungeons and Helltide, without disturbing the core itemization systems.