Aim assist is a big change to a game to me because it necessarily introduces an opinion about how much assistance controller users deserve into the equation.
I get that controllers are not as precise of an input mechanism as mice, but how much worse? How much does having aim assist allow you to play a style that PC players can't? E.g. some implementations of aim assist make quick scoping very strong.
It also just feels really crappy to watch a kill cam and know that if you had done the same thing, you would have missed because no aim assist. The reasons why make sense, it's not a logical thing, but it does feel bad. I would imagine dying to KBM flicks feels equally bad for controller players.
I do think aim assist should exist for controllers, I just don't think it should co-exist with KBM in the same matches in competitive modes. Console players don't seem to like having KBM players in their matches either, so that seems like a win-win.