It's fundamental: any culture which doesn't have a place for families with 3+ children is unsustainable. You can hope to make up the shortfall with immigration,

It's fundamental: any culture which doesn't have a place for families with 3+ children is unsustainable. You can hope to make up the shortfall with immigration, but that's just kicking the can down the road. At some point your have to depend on your own harvest, not the surplus of your neighbors.