> Bitbucket is the obvious existing counterexample: a conventional commercial forge that has never developed into a shared open-source community comparable to GitHub

Agreed. All my customers are on Bitbucket. All the libraries used by their projects (Ruby, Python, JS) are on GitHub.

In the early days of github, it offered private repositories only for paid users. Bitbucket allowed private repositories for free.

I've worked in a couple of companies who used bitbucket, and I've been glad when each of them migrated elsewhere.

Bitbucket managed to be slow, annoying, and suffer more outages than github ever did. (Though perhaps that has changed, we're all aware that github has been struggling "recently".)