You must surely know that people use Microsoft GitHub for far more than source control, right?, with issue tracking, email notification, CI, and GitHub Actions.
I recently tried to get a small FOSS project to switch to Codeburg. The answer was "no" because the free CI for them let them catch some MacOS on Apple Silicon bugs (the devs don't have that hardware locally), and because they are already used to GitHub, making it easier to onboard people and review PRs.
So what's the problem? GitHub gives you a bunch of free stuff that would be pretty annoying/expensive to self host. Seems like a good deal.
If they remove some of that stuff, that would be worth complaining about. Until then, seems smart to take advantage of what GitHub is offering. Their CI isn't even super proprietary, you can basically just write bash scripts.
Quite the opposite, depending on how it’s used it is THE authoritative system for the code, which gets exponentially more valuable with more contributors