It isn't, but Idiocracy while arguably not as bad as humanity dying out is still pretty bad.
If we devolve to the extent of no longer being able to cope with anything more complicated than chasing animals with sharp sticks, that would amount to humanity dying out.
It has been suggested that Homo floresiensis devolved from a bigger brained hominid -- the ultimate in idiocracy.
There is a lot of research on this topic. I know later studies contradict earlier studies, but I'm not sure who to believe here. Wondering if anyone can help.
In case it isn't clear, the basic question is the effect of having children on happiness.