It's not so much the rate of change, but maybe just how much stuff is missing that has to be supplemented by bringing in C libraries, which is totally OK. Rust is going to take a long time to mature, much longer than Go for example, because the project is maybe more ambitious and doesn't have a Rob Pike or a Brad Fitzgerald supporting it and a giant Google behind it. The risk is that it could flounder without critical support, but Mozilla seems to be committed to it. I hope it does well.