I'll wait and see what changes MS brings to GitHub (if any). Worst case scenario would be that GitHub becomes unusable or simply vanishes, in which case I'd either switch to gitlab.com, my own GitLab server or some other public service (kernel.org has a nice list of alternatives
My biggest issue here is that Travis CI basically requires you to use GitHub. Unless that changes, I'll probably keep using GitHub for Knossos/Nebula as long as it's reasonable to do so (i.e. free and not a PITA). If I ever switch to some other service, I'll probably set up a script to mirror all the new commits over to GitHub.
Really, the only thing I'd loose besides the CIs would be the bug tracker but that's easily replacable.
[...] but we may want to discuss how difficult moving to another hosting platform _could_ be if we ever needed to.
Moving the git repo itself is pretty easy. Just update the remote url and push. Everyone else would also need to change the URL or make a new clone. The issues can be exported through GitHub's API IIRC which would take more effort but is still doable. Downloads are already hosted elsewhere, which only leaves Travis which... seem impossible to use without GitHub unless you leave the GitHub repo up as a mirror (but what's the point then?).
EDIT: Looks like AppVeyor works with every public git repo, so we could keep using that if we decide to leave GitHub.