Git is a distributed VCS though. It wouldn't really matter unless your only copy is on Git and isn't cloned anywhere. (In which case, set up a remote repository on Bitbucket/wherever)

If you're relying on GitHub for Issues/Wiki, then fair enough I suppose. Not as easy to get that data somewhere else.

As I understand it, GitHub make the majority of their money from their on-premises 'Enterprise' product, which is sold with a 'contact us to discuss' price tag. Also they raised a huge sum of money while already being profitable. I wouldn't worry about them vanishing.