I can contrast with C/C++ development, where C++17/20 are not available by default on many distros because package managers lag so far behind the curve and new projects get tied to outdated versions.

I just don't really see this as a problem - Rust really isn't that much of a moving target, and the benefit is there is one solution to the problem: use Rustup to manage your Rust installation.

That is significantly more robust than "use your package manager."