This is an important point, but I do believe there's truth to both arguments. The common wisdom in developed countries is that you don't have kids until you're

This is an important point, but I do believe there's truth to both arguments. The common wisdom in developed countries is that you don't have kids until you're financially stable, and increasingly financially stable includes education and career building through your prime reproductive years for both potential parents. And your point, kids were an asset in an agrarian society without machinery, they're not assets in our modern industrialized societies. I think there are more factors, one is the perceived state of the world as a worse place than before, true or not, and another is the increasing control that external entities like the state exert on the internal workings of families. That's not exhaustive, but I do think overall the incentives to start families are just no longer there, so people are doing less of it.