Polygyny advantages alpha males and beta females.
Monogamy advantages beta males and alpha females.
Guess which system advantages civilization?
Maybe that question is too broad. Which mating system — in either the hard or soft forms — benefits the individual? The managerial globalists? The cognoelite? The lumpenproles? Figure out how each group benefits and you’ll know which system is ascendent, and which is actively and passively undermined.