Um, regarding GitLab or BitBucket, I'm pretty sure Atlassian owns BitBucket, and the idea of them being acquired seems hilariously low given they've almost always been profitable and never relied on the pump-and-dump-err-acquired method of Bay Area startups.
It's naive to think that companies will fear having their code stolen by Microsoft.
Companies, which are Microsoft competitors in other fields, use Microsoft software such as Windows and MS SQL Server to process and store valuable data. There was always a risk of Microsoft backdooring their software to spy on competitors.
That doesn't happen because 1. Microsoft wants to be in business and 2. Private repositories probably come with legal requirements that GitHub/MS don't spy on users for competitive gain.
Finally, while software such as Wine has been written to liberate some application from Microsoft's platform monopoly, there is also a lot more Open Source software written for other purposes. I don't see them investing the effort to move away from GitHub unless Microsoft does something evil.
I'll certainly take that bet, if the author wants to back this bold prediction with money.