But unlike many SaaS source-control sites, all your data on github is fairly easily available for bulk download via their APIs... It would of course be annoying to move to a different site, simply because any change in familiar tooling can be annoying, but it's quite doable to automatically migrate everything over.

I think github seems to have been fairly careful toeing the line, and I don't get the sense of being locked in the way I do with some other sites. They appear to be content to compete on the basis of usability and ease of collaboration, not lockin.