If Github were to explode forever tomorrow, active projects would just take their locally cloned repositories, and put them online somewhere else and carry on committing (albeit sans github's awesome social tools). That's the real power git offers us.
It's just a fact of reality that most projects centrally organize through a few bottlenecks. It's easier for people to remember where to go, if there's only one place to go to. But if you're really concerned about it, i suppose you could build a tool to automatically sync a github repo w/ another 3rd party host if you wanted (or visaversa).