they could/want to. AO3 has more legal power and a larger legal team than most people are aware. Project 2025 is still a long ways off, but even if they do "ban porn", there are VPNs, not to mention an entire slew of other hosting sites that authors and readers can move to. AO3 itself is open source, so USA-based users could even make clones of it.

this isn't the first time governments have proposed banning things like AO3, and it's a notoriously hard thing to do. that site is a cockroach among the internet. but if you are concerned, there are options to download your favorite fics for offline reading and storage.

personally, I won't be taking any of my fics down or anything. my fics will remain locked, but if you would like to download them for your own reading, you're more than welcome to. if, god forbid, AO3 actually goes down, I'll move my fics to other sites.

but do what you can to make Project 2025 as difficult as possible. call your senators and representatives, send them letters, annoy the fuck out of them. attend any protests if it's safe for you to do so, and practice safety measures if you do. supporters of Project 2025 are a very loud minority. don't let them drown you out.