Yes. But the current trade-off (performance always wins) means that kernels and other embedded-style programs cannot rely on the compiler doing the "reasonable

Yes. But the current trade-off (performance always wins) means that kernels and other embedded-style programs cannot rely on the compiler doing the "reasonable" thing for UB because it's explicitly allowed to do whatever it wants (which is generally, try for better performance). For kernel-style work, slightly lower performance but predictable/defined behavior for some of what is currently UB, makes life much simpler.