The reason to ditch a specialized product after it is acquired by a large, old user hostile firm is because they have the ability to turn the heat up slowly. They can wait you out.
Doing it before is just premature optimization.
GitHub feeds into that. I'm certain we'll see a "deploy to Azure"-like integration soon.
The 'then' part of that sentence is the issue. It isn't then.
Microsoft are about keeping developers on the Microsoft stack happy. Historically they don't want anyone using any other stack, so they don't want developers making things for other stacks.
Sure, but git repo hosting is a commodity, unlike, say, controlling the Java ecosystem. The full code and history is maintained on all developer machines (at least for the master branch), and the metadata like issues, PRs etc. can be easily migrated off to GitLab.