> Except this is an engine that can be fixed by anyone when it's broken.
Just like XFree86!
Or gcc's failure to be a library!
Fixing a monoculture is bad, in particular when that monoculture defines an interface. It usually takes forks (Xorg) or reimplementations from scratch (LLVM) to get things moving again. Both of these usually take a while before they happen and they take a huge amount of effort. No matter of the original was open or closed.