The kernel is probably too large to rewrite the whole thing at once. This could lead to a future without any new C kernel devs, leading to stagnation, while the Rust kernel could be many years away from being finished. (Assuming we actually move away from C.)
At that point you might as well just start an entirely new kernel and hope it is good enough to eventually replace the Linux one once all devs are gone. Kinda the X11 and wayland thing.