There is no "formula" for success in the creator economy - the winners are largely random. A better way to look at it is there are 4 million humans out there trying every permutation to crack success, and ~400k actually do it.
Unless you have a sufficiently advanced AI agent that is both varying it's content and it's marketing strategy to the tune of maybe ~1000 different iterations it's unlikely we will see a version of OnlyFans that exists that is majority AI generated.
The "parasocial ai girlfriend" sounds like a flawed premise aswell. OF girls are not therapists - Cardi B, Bhad Bhabie, and others aren't raking in millions because they are good girlfriends (although that is part of the upsell). Social status plays a part in the most successful girls, people seem to subscribe because the creator is popular, especially if she's already built a platform elsewhere.
In short, social status does not have an AI substitute.