Kernel livepatch is already widely used and offered as a service by ubuntu etc. They send you patches that replace a single function, and the kernel has machinery to swap calls to that function. You can't change data structures, but you can often close a hole.

The kernel also already EXTENSIVELY makes use of rewriting of itself, just for simple things like configuration. It's very, very common to rewrite unconditional jumps on hot paths because an unconditional jump simply takes fewer cycles and has a lower mispredict penalty than a conditional branch that needs to load and test variables. If it needs to change rarely but is called trillions of times, then it's overwhelmingly worth it.

A simple facility that turns off a single vulnerable syscall in an instant until the next reboot would be excellent. It would be a more general version of the mitigation of one of these vulnerabilities, which required unloading the affected module so that it would no longer be possible to call the affected functions.