So is the solution for women to stop having children and for humanity to die off?

Also, I think the study needs to be much more longitudinal than "2 years on". Arguably the pay off for having children comes much, much later. For some, the pay off doesn't really come until they have grandchildren.

Yes, having children is often a monumental PITA. But humans are social animals, and nothing is really comparable to fill that hole than having a family of your own. The question as posed by the study is silly: yes, you might find less day-to-day happiness if your child is screaming all night long and you need to feed them and change their shitty diapers. But if the question had been 10 years on, and you asked those same mothers "do you regret having children", you would get a very different answer.