> Since it's UB this gives a license to do whatever the f we want, including something that's clearly not at all what the dev intended

That’s really not how it works.

Compilers rather works in terms of UBs being constraints (on the program), which they can then leverage for optimisations. All the misbehaviour is emergent behaviour from the compiler assuming UBs don’t happen (because that’s what an UB is).

Of note, Rust very much has UBs, and hitting them is as bad as in C++, but the “safe” subset of the langage is defined such that you should not be able to hit UBs from there at all (such feasibility is what “soundness” is about, and why “unsoundness” is one of the few things justifying breaking BC: it undermines the entire point of the langage).