> And obviously there are better justifications for this than "we made the semantics of the language easier for programming language nerds to understand".

There are not. For all the noise that we make on message boards about signed overflow being UB, there are very few actual security problems traceable to it. The problems from it are largely theoretical.

That entire blog post talking about the 0.3% regression illustrates just how much of an effort it is, and how unusual it is, to make those kinds of changes. The reason why Google engineers managed to justify that change is that memory safety results in actual measurable security improvements by enforcing spatial safety. Signed overflow being UB just doesn't make the cut (and let's be clear: at that scale the cost of a 0.3% regression is measured in millions per year).