Of course not. But the average woman wants three kids. WHO says babies should breastfeed for two years. My daughter has nursed for 13 months. A mother taking off three chunks of 13 month leaves while married to a man probably earning as much or more than she does (since women disproportionately marry similar or higher earning men) is going to -- at some point -- realize that it's easier to stay at home. That is exactly what my wife did.

> Even the concept of the nuclear family - with one possible nurse and mother figure - isn’t a universal construct of human societies.

Yes it is. The only people who claim this are people who've never lived in an extendend family situation. You still know who your mom and dad are. They are still the primary care givers, and babies still have a primary attachment to their moms when born that self-reinforces itself unless there is some major disruption.