I don't know why you're trying to explain an outcome that is opposite of the observed outcome.
Are you saying that while more money is correlated with less fertility (the fact), that somehow even more money will reverse the trend and start going the other way?
Based on observed data, one could almost make the case that if only billionaires start stealing from the poor even more, then birth rates should go up.
I've always believed that it isn't money itself, but access to healthcare and education that lower fertility rates. Money correlates really strongly with access to healthcare and education.
Having good education and healthcare leads to birth control and maybe an abortion if the BC fails.
When a family of three is making $15K a year, do you believe baby four, five and six were planned out in agonizing detail? Or do you think maybe they weren't planned at all?
And I do believe more money going around would help. If the perspective is "I can't afford it", those who are doing OK will not have children if they can help it.