There isn't very much lock-in other than the domain names - Fandom wikis are all built on MediaWiki, and they have all of the MediaWiki export infrastructure (which exports everything about a page, including history) still intact last I checked.
Fandom's lockin is mainly not being able to migrate your community (they will ban you if you link to the new community) and having more SEO juice than you.
Wikia/Fandom has a history of refusing to delete wikis where the community has agreed to migrate (such as when many wikis left after the forced skin change in 2010), leaving a stale copy with better SEO. I don't know whether they still do this nowadays, but after that incident I swore never to contribute to Fandom ever again (for new wikis I try to recommend other wiki farms, usually Miraheze).
Someone will copy your wiki's content into Fandom and Fandom will starve you voa SEO.
If that's really happening, then DCMA take-down?
Most Wikis use permissive licenses. Also, has anyone actually