I'm not convinced that there are really two distinct sets of people in the world: stupid and not stupid. In my experience, most people are pretty good at some things and stupid with other things. I notice that in myself to a certain extent, and forces like the Dunning-Kruger effect probably prevent me from noticing more.

In terms of the original article, I think that answers the question of why stupidity continues to exist. Most people (even ones who appear to me to be stupid) are smart enough at enough things that they continue to live and procreate.

The game theory examples the author gives in the second half of the article over-simplify the issue by assuming stupid is a binary on/off attribute. Within a game, you can assume that's true by assuming the people make stupid decisions within the rules of the game. But in real life, there isn't one single set of rules that determines whether we or not we live and procreate.