I don't think humanity survives if everyone just has one child.
I don't think it's terribly surprising that there are diminishing returns on parenting time and resources. The question is whether it's linear progression or if there's a drop off past a certain point.
I would be very curious if there's an optional number of children revealed by this research.
Edit: Very curious why I'm being down voted on this. I'm not espousing everyone have a lot of kids. I'm just trying to reckon with the tension between optimal parenting and the ongoing existence of the human race.