Just imagine how good this game and community around it would be if it had always been open source. Just imagine how much money they could raise in a Kickstarter to simply open source everything.

I don't think he'd even need to even open source it, in terms of allowing anyone else to release it. If it was just possible to hook into it and make custom UI or Renderers, I think people would run with it. Some have tried but they are extreme hacks at this point.