UB doesn't mean there will be nasal demons. It means there can be nasal demons, if the implementation says so. It means the language standard does not define a behavior. POSIX can still define the behavior. The implementation can still define the behavior.

Plenty of things are UB just because major implementations do things wildly differently. For example:

    realloc(p, 0)

All of that implementation freedom is also available if the behavior is erroneous instead. Having it defined as UB just gets you nasal demons, which incidentally this rule leads to on modern compilers. For example:

https://godbolt.org/z/ncaKGnoTb

Yeah that’s just really bad language design. Which, again, literally no modern languages do because it’s just terrible horrible awful no good very bad design.