Memory leaks are not uncommon on GNOME, and i assume on other desktops as well. But Rust is not just good for preventing memory bugs: strong typing prevents lots of logic bugs as well.

Also, rust's "fearless refactoring" paradigm enables for incremental optimization which i've found much harder to deal with in other programming languages. I think that's a point worth considering for a big piece of engineering like a desktop environment.

EDIT: Also memory bugs are not just "memory leaks". There's plenty of subtle bugs introduced by off-by-one errors and undefined behavior. Just because memory usage doesn't rise unexpectedly doesn't mean software is working as intending. I don't know about you but i personally have had weird bugs with all desktops i've tried so far.