> Instead of just "builds with gcc" we would need to wait for Rust support.
There's always rustc_codegen_gcc (gcc backend for rustc) and gccrs (Rust frontend for gcc). They are't quite production-ready yet, but there's a decent chance it's good enough for the handful of hobbyists wanting to run the latest applications on historical hardware.
As to adding new architectures: it just shifts the task from "write gcc backend" to "write llvm backend". I doubt it'll make much of a difference in practice.
This will be an impediment for new architectures in the future. Instead of just "builds with gcc" we would need to wait for Rust support.