I think its less about affordability and more about basic choice. Women have choices these days over their bodies, over their lifestyles, over their relationships. Growing numbers are happy to say they only want one child, not because they cant afford a second child, but because they dont want one. Or even dont want any at all.
The very large families I work with all come from the lower socio economic groups, those on benefits or very low incomes. Yes there is a catch 22 because if you're on a middling wage then you get less help but still there are free nursery hours and we have free at the point of delivery education and health, something a lot of countries around the world dont have
The undesirable truth is that humans may well die out over time, not all species survive. There are too many humans in the world anyway.
And as a few posters have identified, why the moral judgement on people choosing money over children anyway, if thats whats happening (which Im not convinced of). There's no 'rule' that says that humans have to desparately want family life.