This bullshit excuse that somebody can't afford to have kids is proven wrong by the fact that poorer people have more kids than rich people. You can even be unemployed. Gone are the days of destitute single mothers having to give up their child to the church and work in the poorhouse. We have social welfare for that.
Maybe the fact that poor people can have lots of kids has taken away their value as a status symbol for wealth?
There's a well known aphorism about this...
In the past both poor and rich tended to have tons of kids, this is because kids tended to die young regardless of being rich or poor.
Then you're trying to compare massive social changes in the west that occurred around the same time. For example womens suffrage, women being allowed to work, sexual revolutions and birth control.
If you look at countries that tended to develop later, the rate of childbirth tends to drop with accessibility to birth control and education.