David says that even if you were to fork Ruby on Rails and call it something like Snoopy on Pails, the chances of your project sticking around are very low. The "gravitational pull" of the Rails community would ensure that more commits are done to the parent master, than to the new fork. So although technically open source, project that gain momentum also gain dominance. Even if that power is somewhat distributed, it is still located mostly with the company backing the engine.