The issue there is interaction between exceptions and deterministic destructors. In the end it means that there has to be some hidden code for every stack alloc

The issue there is interaction between exceptions and deterministic destructors. In the end it means that there has to be some hidden code for every stack allocated object whose destruction could be skipped over by exception handling mechanism. There are various way to implement that, but all of them involves something that is "hidden code" and also the mythical "overhead for feature you don't use". Another giant issue with C++'s "if you don't use a feature you don't pay for it" and interactions of various features with each other is that features you do use tend to be horribly expensive. It seems to me that authors of C++ tend to use some antiquated model of what "runtime cost" means which totally ignores effects of cache locality and code size.