Which makes building a community the most important thing to do, to make it harder to keep closed forks closed. It's worked really well for LLVM/clang, with contributions coming from a number of normally open-source hostile sources. Conversely, were Microsoft's engine here GPLed, with the contributor licensing agreements open source organizations like Canonical and the FSF have, it would be a lot easier for them to close things back up, and take all the toys with them.