If Mojo's goal was to make evolution easier, they would have a limited closed beta like what Jai does. But they're not doing that; they make the toolchain available to everyone, they just did this marketing push around releasing version 1.0 of their language. They're making it harder to make breaking changes by seeking users among the general public.
I have no idea why you started talking about outside contributors though, nobody's arguing that they should accept outside contributions. You do t have to do that. It's a completely orthogonal issue to what we're talking about, which is releasing the code under a FOSS license.
I feel like they're going to remain closed until Qualcomm is convinced they can't make any money from it being so.