These two projects are older than Rust itself, and have hundreds of person-years of effort put in them. That's a huge investment, and huge inertia. It will take a similarly long time and large investment to replace them. It might happen eventually. Blink is adding support for Rust dependencies. Rustlang is adding pure-Rust cranelift back-end.

But also look into the future: if someone is currently writing very first lines of a project that will become equally big in a decade or two, that may likely be in Rust, not C++.