At scale, every single one of these other platforms are going to run into the same issues except Microsoft has the money, know how, and capability of believably sorting this out at some point.

(I said "believable", not "realistically")

So the real reason GH continues to grow is that betting on GitLab just means that you'll probably run into the same problem at some point, except they aren't a hyperscaler and if even Microsoft has to span their workloads into AWS to scale, what hope can you have with smaller vendors?