But then we end up with the OOM Killer, which is awful - randomly kill the biggest process because "reasons".... It would be better if the OS could say no.
https://unix.stackexchange.com/questions/282155/what-is-the-...
Without over-committing, you could be preventing something that would work anyway. With it, the OOM could (often does) pick the biggest process that could very be using its memory legitimately AND be the most important process of this machine too.