> The instability of Rust would not affect users in any way.
Sure it would. Suppose a rust-based package has a security bug. Upstream has fixed it, but that fix depends on some new rust language feature that the frozen version of rust in Debian doesn't have yet.
Then the responsible Debian maintainer would backport that fix, as they have done in other languages for decades. Really, that's not user facing. It's a possible hassle for the maintainers and developers, which might be bad enough, but not a problem for users.