The problem with polygyny is that it creates a few very reproductive chads who are highly incentivised to protect society and work to sustain it, and a LOT of incels who have the opposite incentive.

It's why countries like pakistan end up so rapey and backward: it's in most men's interest to work as little as possible and take women by deception and force.

Then eventually they come up against a highly productive monogamous society and get steamrolled because the other guys have thousands of mids working their ass off and churning out steel weapons because they get laid every single week without fail and have five kids each.