Cost of migrating from GHE to something else usually exceeds the cost of GHE itself.

Migrations generally are non trivial. They don't generally happen just to push one's principles.

Depends on how you do it. Of course if you want to move instantly then that's gonna be hard. But slowly moving isn't that bad, or creating new projects somewhere else and slowly porting old ones.

Even then, it's possible that just companies that planned to buy GHE will simply not buy it and use something open source instead.