I think it is at least worth considering that there may have been reasons other than economic reasoning that people chose to have children.

And apparently that results in people having 1.8 children instead of 5.6.

I’m not sure what you’re saying. There are other factors that have changed between now and then, such as the general acceptability of being unmarried, family planning tools, reduced religiosity, and woolier stuff like people’s idea of what a good life is.

Except the linked article argues that in hunter gatherer societies, kids were a net drain even when their parents were approaching retirement, and in agricultural societies there were "retirement" options in the form of land ownership and financial instruments.

Some of the calculations they made were utterly bizarre. Like assuming the child wasn't paying anything forward until "retirement" of the parent. All adults being net positive is not the same as the elders not benefitting from in-kin capture of investments in a scheme that almost resembles taxation moving benefit upwards in in-kin seniority.

So if you were living in an agricultural society, you would trust their financial instruments and institutions governing land ownership more than you would trust your own family? Really?