Yeah, but that's until some library makes a backwards-incompatible change and the whole things stops building. Sure, it's better than a completely proprietary service, but support and active maintenance are very important too.

The Rust ecosystem and build system, is designed to prevent this. If you're using a library and want to code to keep compiling in 10 years, you can (and should) just lock the dependencies.

Since the libraries are all mirrored on crates.io and, by design, cannot be removed once they are published, you should be good, at least until the day the owners of crates.io stop being able to pay the server costs.

I believe that you can also host local mirrors, but I haven't checked that.