Surely you don't think that's a bad thing! I'm sure even Rust's most vocal proponents are aware that no one language (at least not this early in the history of computing) will become the "one true language" (even for a given design space) indefinitely into the future.
That's...a really fascinating idea, but one that, at first glance, seems to have only downsides. Backwards compatibility is certainly a pain, but it appears to be a sine qua non for industry adoption. When you say "dropping source-level compatibility", do you mean that some kind of lower-level compatibility (e.g. with FFI between different language versions) should still be maintained?