In my situation, the cost to feed/clothe additional children isn’t preventing me from having more kids. It’s the cost of childcare, and the cost of college. Assuming it’s $10k/year per child for childcare from the age 0-12, and you have multiple children in the age bracket, your income needs to be extremely high to afford. Even a state college at $20k/year, with multiple students, is going to strain parents anywhere in the bottom 99%.
As unfortunate as the lack of economic mobility between bottom 20% and top 20% - it exists. So, children born to the top 20% are more likely to have future incomes in the top 20% - and more likely to be net contributors to Social Security.
It’s hard for politicians to fix the mobility between income brackets, but it’s easy for them to alter the tax code to incentivize higher income families to have more children.