Do you imagine how long it would take to compile the Linux kernel if it were rust only? Not to mention the kernel has to allow for third party closed source stuff like drivers, wouldn't that force you to allow unsafe Rust and put you back to square one?
That seems an insignificant price to pay if it would truly provide the promised benefits (big if). Even most Linux users don't compile the kernel themselves, and the 5% that do care can afford the time and/or computing resources.
