GitLab could function (at least somewhat) as a drop-in replacement for GitHub, as existing GitHub API client libraries could point to GitLab instead with minimal/zero modification.

Interesting idea.

I'm afraid it would cause us to follow their changes, which makes it a brittle strategy that would prevent us from innovating. We prefer people to adopt GitLab itself.

You could provide an adapter API which translates Github-like requests to your API. If you want to use specific GitLab API features, developers use the "native" GitLab API.

I mean, if anybody really wants that they could easily open source it on their own. : P