I think the work being done in getting an AI to generate a story is fundamentally different from the work done to write a story without one, and it's dishonest to claim authorship as if you'd written it. (But there's a fair amount of wiggle-room in between, it's a gradient not a line.)

I think there are major ethical (and legal!) issues with how the current large machine learning models are getting and processing their data and how they're making them available for use and all that's going to be a mess for a long time.... but that we're not far off a situation where there will be one that doesn't have those issues and is still capable of doing something like writing fic, so it's not an answer to the deeper question.

I do think it's a bit pointless to post AI-generated fic to something like AO3, because presumably if someone wants an AI generated fic for their fandom, they can generate their own? Like, I feel like, once readable AI-generated fiction becomes easy to get, it also becomes disposable in a way human-made isn't. There will probably be a transition period while people figure that out, though. (If someone puts enough human work into an AI-generated artwork that the average schmuck can't get something just as good with very little skill or effort, though, the answer's a bit different.)