You misunderstand completely. Of course small random projects like flappy-bird-in-cobol may benefit from a migration. I'm talking about well established project

You misunderstand completely. Of course small random projects like flappy-bird-in-cobol may benefit from a migration. I'm talking about well established projects. Also, the metrics you propose are quite meaningless. There are contributions that actually waste core developer's time but are grudgingly accepted either for political reasons or to shut up the proponent. These kinds of contributions are certain to increase by a migration to GitHub.