I've rewritten two personal, non-trivial projects in Rust and both have been successful. The most recent Python -> Rust conversion did take a bit more time than expected but it's stable and has already saved me a few hundred in AppEngine hosting bills.
I'd be interested in some elaboration. How did switching languages save you money?
Presumably the normal way (since they're talking about hosting bills). Rust is faster than Python assuming you don't go out of your way to make an inefficient program, and typically consumes fewer resources (though not always, but for roughly equivalent programs it should). This means you can get away with either fewer instances to perform the same amount of work, or the same number instances but for less total time. (Which benefit you get depends on the style of your system, but both will save you money.)
I moved from AppEngine hosting to self-hosting (on a Pi). The server was expensive to run on AppEngine and effectively free to run at home. The CPU barely ticks up, even when running a load test.
I didn't realize that AppEngine supported Rust. Are you compiling to wasm and then using their JavaScript runtime?