I think it's fine for Guix to use a specific version of Rust internally, and to be conservative about updating that.

But Guix becomes unsuitable for anyone developing software in Rust, if this technical detail in the package manager means that they can't use newer versions of the toolchain.

I think it would be best if Guix separated these concerns, by disconnecting the "Rust we use to build packages internally" with the "Rust that gets installed with `guix install rust`"