It is just a git repository. Sure, you may lost some issues if it is stuck down and no one made a backup, but the important part, the code is now spread all over the internet. Should they want to migrate, all they need is to ask those who used GitHub as the origin to do a "git remote origin set-url".

That's one of the best things with git (among with other DVCSs), it is incredibly resilient. I remember during an interview of Linus Torvalds about backups, he says he doesn't backup his work in the traditional sense, he has no real reason to when everything he does is copied millions of time.

The real point of GitHub for such a project is because GitHub is really popular, and it has a social network aspect to it. So yes, you are going to lose it if you get kicked out of GitHub, but by choosing a lesser known platform you won't get it in the first place. Also, their website is not on GitHub, so kicking them out of GitHub won't make them disappear completely.