Not necessarily; the compiler has to trace ownership anyway and could also detect "legal" use of pointers and reject certain uses for objects under the "borrow

Not necessarily; the compiler has to trace ownership anyway and could also detect "legal" use of pointers and reject certain uses for objects under the "borrow checker regime", as Rust does it. But of course one could take a subset of C and augment it with the new features, as it was e.g. done in C++ in the eighties or Simula (with Algol) in the sixties; the obvious advantage is the similarity to the original language and (potential) reusability of a large existing code base.