I am not convinced that anyone would take a working rust project and rewrite it in C. I don’t see any good reason to do so.
When rust will lose popularity, it is going to happen eventually, I would bet it’s in favour of a newer and more promising programming language. Not C.
new chips will always have a c compiler available long before anything else
Compile speed maybe the only one. But hopefully that keeps becoming less of a difference
I think Rust has hit critical mass. It's now basically the default choice for something you want to perform well but want to be reasonably secure. For example, uv in the python ecosystem.