Massive language changes are easier to do without users. Whether the source code is visible to external people or not is completely irrelevant. I have a few programming language implementations on my GitHub and let me tell you, making manor breaking cha he's to those are easy as pie when I'm the only user.

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.