Microsoft survived on inertia. Windows was the default. Office was the default. GitHub, after the acquisition, became the default forge for software teams. That kind of position lets a company coast for a long time.
That's how I remember it from before I switched to Linux.
Just one off-topic niggle: when Github went M$, Git was already a big thing, and other big Sourceforges supported it, maybe most notably GitLab (I considered moving from GH to GL but GL was already way too corporate). Sure, MS envisioned GH to be "the default", but it never really was. The biggest, probably, but never the default.
And if you think that all repositories are on GH, check how many of those are actually just mirrors.