Sometimes different vendors provide some benefits. For example Intel's C++ compiler produces (or used to produce?) much more efficient numerical code than either gcc or clang.

So for numerical applications C++ may make more sense than Rust. Rust does have the advantage of being based on an LLVM backend. So perhaps different vendors can compete by writing more efficient backends that are applicable to both C++ and Rust (but you probably lose some information when skipping the compiler front end)