requiring a single commercial entity like github is a really bad idea. Sure it'll be fine for a few years but when github goes belly up or gets bought and the new CEO cuts all extraneous projects, then you're hosed.
That's counting on Microsoft taking good care of GitHub and maintaining its goodwill.
It might or might not be well managed in the years to come. Case in point, Skype. It was "the" tool to use for office communication. Hardly anyone I know uses Skype nowadays.
Sadly yes, because big businesses with insufficient concern for maintaining openness on the internet keep choosing siloed proprietary communication services, just because they're "more polished".