Thanks for mentioning GitLab as the next big one. We're seeing people #movingtogitlab
The bigger question is why GitHub didn't IPO. They have name recognition and prominence such that it wouldn't really be a surprise to hear that they were doing so. I don't have any internal details, so maybe the numbers just didn't line up, but that would've been the investor-approved exit that allowed it to maintain some neutrality.
That leads to the next question: if GitHub didn't/couldn't IPO with all of its name recognition, why should anyone expect that GitLab will?
While Atlassian's open-source pedigree is much weaker, they own BitBucket, and they're a bootstrapped startup that focuses on developer tools. They IPO'd a few years ago. They're probably the best bet for developers interested in a "neutral" hosting platform.