> capitalism solved the problem by making us all too poor and miserable to have children.
Nah, actually most of the birth-rate decline is because women delay children because of a career. e.g. imagine if 20 used to be the median age for kids but it eventually hits 40, with a constant life-span of 80 years, everything else being equal. In the 20-year generation scheme, you average 4 concurrent generations alive but in the 40-year generation scheme you only average 2 concurrent generations alive. e.g. the long-term stable population would be half what it would be, if the women had children earlier.
The result is that the stable level of the population fell by half, even though when you check the average fertility rate it didn't go down, because fertility rate only counts the average number of children per women per lifetime, not how old she was when she had them. A shift from a norm of 20 year generations to ~27 would amount to there being 3 concurrent generations instead of 4, so a drop in the stable long-term population level by 25%. e.g. to control population levels up or down you don't necessarily need to make people have more or less babies, you just need to incentivize earlier or later births.