I believe Rust is too young a language to replace C in Emacs. I'd be happier if it was a standardised, time-tested language with a large user base and lots of docs.
Rust is standardized with lots of docs. The rest two will come with time.
It's still moving fast and breaking things in ways that---while necessary---aren't reasonable choices for programs like Emacs or TeX. These programs need to run the same way in 20 years, and be runnable the same way in 50.