The other direction might be more interesting, in case rust drops in popularity in a couple of years, leaving behind a bunch of "let's rewrite in rust" efforts
That would also help use Rust in platforms that only have a C compiler.
That could be interesting. If some new language or tool appears that automatically figures out the correct lifetime and ownership of the resources in your program, people (might be the same people) will call for rewrites from Rust into the new language, as you would no longer have to assign memory ownership manually.
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.