The self hosting will still be there and working as expected no matter what GH does (fails... again, DMCAs the repo, bans the account, etc.). Self hosting isn't only about being the only one with the data, it's also for the independence aspect. GH as a backup doesn't hinder the independence. Network effects are strong and make a lot of developers still have a GH presence as a secondary platform.

The evolution is when one can finally fully disconnect from GH, the main self hosted platform will continue to operate as if nothing happened.

A migration can have a period of parallel running.