One of the best cases of leaving Fandom that I know of is the Runescape wiki mentioned in the article[0]. The community that ran the fandom wiki had buy-in from the creators of Runescape to assist with the transition, help with funding, and eventually direct integration into their games. In a game as information-dense as Runescape, that updates weekly, the wiki is basically a necessity for folks to play efficiently, or to find out how a new update actually works.
Fandom isn't the only bad wiki site though. Fextralife had (has? I haven't kept up) an issue where they were embedding Twitch streams on each page load, which was boosting viewer counts for whatever streamer they decided to embed on every page.
I'd love to see a world where more companies self-host the wiki for their game/TV show/etc, especially given the relatively low cost of deploying and hosting, but I also understand that most companies don't have the motivation to do that as it doesn't always directly impact their bottom line and it can take effort to moderate.