I would speculate that the disincentive is larger for wealthier people. A lot of child-related expenses are for positional goods (house in a good school district, saving for college) where the price is basically "all your spare income". So if you're rich having kids means giving up awesome vacations and the option of retiring before 50; if you're poor those things were never on the table.
Yet it's low-income households who have the highest birth rates [0] afaik this isn't unique to the US, in Germany the situation looks similar.
It's extremely interesting how this relation between income&fertility doesn't just hold true on a country vs country scale, but also on a domestic level inside these countries, there must be something very fundamental at work here.
[0] https://www.statista.com/statistics/241530/birth-rate-by-fam...
Once people can choose to not children, factors like the economy affect population growth.