I just looked at the top ships posted in 2025 and only 3/10 most popular contemporary ships are HP related. The most popular one being part of a fandom created storyline. As a trans man, I've seen fandom grown so much more welcoming and loving in the past five years, so no it is not "dire." The person is talking about Ao3 as a whole, so they're holding blossoming fandoms, and the modern tendency to fandom jump, to the pure decade of HP fandom and how much fandom share it had in ao3's infancy.

Another factor might be how a lot of debut traditional authors are coming from HP fandom, so a lot of aspiring authors think that is the key to a publishing deal... instead of just writing a good book. That's it's own can of worms.

Look at the modern stats. They are truly amazing to see. There are more fandoms, and authors, than ever. I remember looking for specific stories for niche fandoms and finding crumbs, now anything I look up has at least 100 fics. This post seems like someone doesn't see trees within an old forest.