> There's definitely major benefits for diversity in this area (meaning not having the vast majority of projects on one platform), but I'm hoping we (as developers in whole) don't throw the baby out with the bathwater here.

Perhaps tooling can help with this. Github, Bitbucket, and Gitlab (and I presume some of the lesser used solutions out there) all support some type of forking and pull request model, even though that's not core git. An abstraction layer atop that can hopefully obviate a hard dependency on one platform.