Yeah, people forget that two classes have been playing with permanent (limited) AI companions all this time (and one other spec since 4.0)
Some even tank for you!
And you bet that any pathing improvements that come with the new AI will be applied to player pets as well.
I don't think anything the new AI accomplishes will be comparable to human players and thus be able to replace them.
It basically doesn't go beyond scripted reactions and movements - and BfA mobs already have abilities to port behind you (quite annoying when you're playing half-afk, just going through your outdoor "rotation")
It won't be much more than upgraded Bodyguards and
maybe a way to fill up premade groups for trivial content = normal islands/dungeons. E.g. when your group has 3-4 people and healers are so rare in LFG that it triggers
CtA for that role, you have the option to take an NPC healer along. The NPC has a basic "if damaged, heal" ruleset, maybe throw a CD on people standing in fire for more than 1.5sec