The long-term goal is for Rust to overtake C in the kernel (from what I understand

Your understanding wrong. Rust is limited to some very specific niches within the kernel and will likely not spread out anytime soon.

critical code gets left untouched (a lot of the time) because no one wants to be the one that breaks removed

The entire kernel is “critical”. The entire kernel runs - kind of by definition - in kernel space. Every bug there has the potential for privilege escalation or faults - theoretically even hardware damage. So following your advice, nobody should every touch the kernel at all.