The cost of raising a child is basically the socially acceptable (but untrue) excuse that people get to use.

It is pretty easy to look at millionaires and billionaires and see that money isn't what is holding down birthrates.

Just look at the millionaire CEO of your own company. Do they have 6 kids? They probably don't even have 4 kids.

And there have been studies (though I don't have any at hand) that sudden exogenous wealth (e.g. winning the lottery) doesn't really lead to people having more kids like they claim they wanted to.

Or do cross-country comparisons. The US birthrate is (very roughly) the same as countries that have (even adjusted for purchasing power parity) dramatically lower incomes.

Or look at low engagement levels of grandparents nowadays despite being dramatically richer than in previous generations.

It isn't (mainly) about money. It is just that it is okay to complain about money but not as okay to complain "I'd honestly rather do my hobbies than have another kid".