First, Debian is not a distro where users have to compile their software. The packages contain binaries, the compilation is already done. The instability of Rust would not affect users in any way.

And second, as a developer, I never had a more unpleasant language to work with than Rust. The borrow checker back then was abysmal. Rust is not about developer happiness - Ruby is - but its memory safety makes it a useful option in specific situation. But you can be sure that many developers will avoid it like a plague - and together with the breakage and long compile times that's probably why moves like the one dictated here are so controversial.