See? I knew someone would mention Possum.
They always do

Possum is (was?) a commercial project. With a changing community, a "Community Effort" would make everyone's lives very difficult. Getting 1000 people to sign a contract, for a start. And then everybody would get peanuts.

Of course, a non-profit project could work. But the JJ2 engine is complex.
Even if there were enough people, with enough patience, they would probably want to start a whole new project. With the file format issue it would simply be easier. And if they did that, they'd eventually make it commercial.

So we have a vicious circle here, folks.