Not only that, the code was wrong. The specification is quite clear that correct programs don't cause UB to be executed at run time. If your wrong code now produces wrong results, that's because it's wrong. That your compiler allowed you to get away with it for decades is a compiler bug, not a feature.

Do I fully believe all of the above? Not exactly. But compiler authors do. Does it make a really good argument to never use C or C++? Yes. If only we had 50 years of optimization work in any language with better semantics.