I'd be fine with AI doing ranking. Isn't ranking just checking if circles and sliders aren't overlapped and that the map is otherwise playable? If it checks to a spec, I don't see a problem.
For AI making maps to get ranked, I'm fine with that too as long as it produces playable difficulties that aren't just confusing sliders and streams upon streams for "difficulty" inflation.
I've tried making beatmaps and getting ranked in the past and it was too much effort and having to beg others for feedback. Wouldn't even consider doing it nowadays and there's plenty of songs I'd be interested in seeing mapped. Passing that effort off to AI to make something fun would be great! I don't care about legendary mappers or whatever, I want to click circles to music, drama-free.
Heck, maybe just integrate AI in osu! to automatically make beatmaps with just a song file. Create algorithms for different presets as to what kind of fun or difficulties you want, and let the AI do the BPM, distance calculations, and whatever else. This is the one case where I'd actually like to see AI take something over :p
Edit: Thought about it a bit more, and implementing AI like that would likely ghost-town communities and any real goal of osu! gaining popularity or future support. That's definitely a worse outcome!