Rust does not safely support all of the things folks do in C++ (complex cyclic object graphs with inscrutable logic deciding lifetimes, intentional races, etc).

The whole problem is that C++ doesn't support them safely either. The committee has no interest in fixing C++, so what's the alternative that doesn't involve a new language? DARPA already considered and rejected things like sandboxed runtimes and rewrites from scratch because they don't fully solve the issues.