They don’t actually let admins shut down their wikis or remove content from them. They can leave and start a new wiki, but they have to leave the old one in place (for which Fandom could potentially just find new admins), and they can only link to the new wiki from the Fandom wiki for a period of two weeks. With Fandom’s SEO, there’s a good chance the Fandom wiki will still be ahead of search results of a new wiki even after migration. Source