This exact problem exists today if someone takes code from the Linux kernel that is compiled with things like -fno-strict-aliasing. Undefined Behaviour makes programs unpredictable in the first place, given that you can't reasonably ensure you're not triggering it. I would expect a lot more people to use friendly-c (or simple-c or yes-this-is-how-your-cpu-works-c) than standard c.

As he notes, people who need the undefined behaviour-induced optimisations are in the minority.