Honestly, with the kind of changes and restrictions that you would end up needing, you'll end up with enough of a different language that you can't cleanly migrate a big existing codebase.

If a codebase wasn't designed with the intent of being able to prove validity & safety in the first place, trying to achieve it would be pretty rough. Not literally the same as trying to rewrite it in Rust, but the biggest problems would probably be comparable.

It is still easier than a full rewrite.

Adoption of C++, Objective-C over C, Typescript over JavaScript, versus alternatives that forced full rewrites, should be taken into account.

It is no accident that one of Swift's long term roadmaps, is having for C++ the same kind of transparent interop they leverage from clang with Objective-C.