GitHub is certainly overpriced for its private plans, but I don't think releasing the source would destroy their profitability. Once the ball gets rolling on something like GitHub, network effects play a major role in its continued usage and popularity. Good support and constant evolution of the platform also encourage users to pay for your hosted service even if the code is available elsewhere. GitHub's features like pull requests, profiles, and expected presence on resumes as the de-facto standard "git hub" (har, har, har), and overall convenience would continue to promote usage of github.com.