> But the major players do seem to be happy to replace their C++ code with Rust.

Because incremental improvements don't provide enough value. A stable C++ codebase is best left untouched. It's not worth to rewrite C++ into C++++ to get a couple of features that are flawed retrofits backported from modern languages. In the end you still have C++.

Haskell has a pretty good story about how to use different dialects in the front end of the language, as long as the backend is still the same. Well, when I say Haskell, I mean ghc.

Alas, C++ still mostly does its libraries via conceptual copy-and-paste (#include). So that would need to be fixed, if you wanted to mix-and-match dialects. You could probably keep '#include' syntax, but subtly change its meaning.