Optimization often requires knowledge of the behavior at a level of abstraction lower than the one you're currently working at, and exploiting leaky abstraction

Optimization often requires knowledge of the behavior at a level of abstraction lower than the one you're currently working at, and exploiting leaky abstractions. Taking away the assumption of two's complement makes code more portable at the cost of making a lot of optimizations impossible. It's a tradeoff, not an absolute rule. As soon as undefined behavior is guaranteed by a spec, it's no longer undefined, so your statement reduces to "you should never rely on UB." You can say that, but people will still do it because it makes their programs faster.