You really don't need to get so elaborate. The shift from agricultural to industrial/service economy explains it well enough.

In an agricultural economy, children are an economic assistance, a source of labour, and a means of helping with survival.

In our industrial/service capitalist economy, while they are a net good for society ... they are a cost centre for the parent.

Yeah, as soon as you don't need children to help with your work, they don't make much sense in the capitalist individualistic society. That women still choose to do it, honestly... I see as a triumph of the human spirit