> Also, C is extensively standardised

Not really, at least not in the sense that you mean here: The generation of machine code in shared objects. There is a distinct lack of definition between C syntax and the resulting machine code.

> The best advantage of writing libraries in C is that it allows the library to be usable via FFIs in many other languages.

You can create C shared objects from Rust (or from a ton of other languages which generate native code). No big deal. It can do what you want.