There are already alternatives to Fandom. I volunteer with Miraheze.org, which hosts wikis without all of the ads, has been up since 2015, and has recently become a US nonprofit. We're currently geared towards more technical users, but we're trying to make more tooling to simplify wiki administration as we've taken on more users.
I also hear wiki.gg is pretty good, but they're focused on gaming wikis. A lot of Fandom wikis jumped ship to them (of course Fandom is like Hotel California, you can check out any time you like but your wiki content will never come down).