Fanfiction is such an "extreme niche" that AO3 is #53 globally, FanFiction.net is #572, and Wattpad is #323 by Semrush lmao.

I'm not sure what your argument is. I don't have anything against fanfiction, but a couple of sites with a few million users falls under my definition of niche. "Extreme" might be taking it a bit far, but it's a long distance from mainstream.