Standard enshittification: They've been pushing features nobody wants and making user unfriendly decisions. I'm sure other people will provide specific examples, but that's pretty much it.
Another thing is that, due to their market position, they won't let people use alternatives. If there's a big enough fandom wiki, google pushes it to the top above any independent fan supported wiki.
Not even unreasonable. Straight up hostage. I'd you post a wiki to their site thry effectively own it, and if you try to close it they will simply steal control of it and refuse to delete it under any circumstances to continue serving ad revenue. Which is what happened to fucking Mojang with the Minecraft wiki.
LPT use the Indiebuddy wiki to purge all Fandom links out of your browser and replace them with the correct indie wiki.
So what you are saying is that you should make a script that slowly changes random articles on a wiki on fandom, so that over time the information will be gone and any backups fandom may have taken are outdated or bad as well.
EDIT: this reminds me of a virus I heard about, it would infect your excel files, and slowly introduce rounding errors in them.