The FSF does not consider Github to be a suitable VCS host for its projects. Gitlab has, to my memory, a rating of "good enough", but only Savannah (which the FSF hosts instances of) is truly thought of as excellent, and it's what's used for all the current projects under their control.
If Magit were to become a FSF project, it would be made to migrate.
https://git.savannah.gnu.org/cgit/emacs/elpa.git/tree/packag...
https://github.com/magnars/dash.el
> All the Elpa packages are hosted on a Savannah instance.
That's the "publishing repository". You have to push there for a new version of your package to be published on GNU Elpa. But for most packages development happens elsewhere, and in most cases "elsewhere" is Github. Emacs maintainers sometimes commit directly in the Elpa repository (and package maintainers are expected to deal with the consequences, such as "two diverging mainlines"), but development happens elsewhere.
Is a model like this what you mean when you say "No, we would continue to use Github for the foreseeable future"? I guess if you're doing all the legwork, it won't be overly visible to other people who want to contribute. Still though...