Ignoring the fact that the VCs want to get paid and GitHub's employees want to keep working, how big of a skeleton crew would it take to keep a site like GitHub running in "maintenance" mode?
Maintaining the app and not moving forward with new features and adding value (because of lack of resources), I would assume it would just die eventually.
Doubly so when a brand reaches infrastructure level - like Github, or arguably Dropbox. Just don't screw with the product that's working well. Not breaking things isn't expensive.
It's the corpse model of product development.