Right. Kids cost time and money. So the lifestyle you can afford _with_ kids is slightly lesser than _without_ kids. But this is true at all income levels below the richest 1% or so.

Every person who chose to have kids had to make some lifestyle adjustments. If "we cannot afford kids" just means "if we had kids we'd have to make some lifestyle adjustments" then practically nobody could afford kids in this sense, including the overwhelming majority of people who _did_ have kids and are doing fine.

That shows that "we cannot afford kids" is not really the reason you're not having kids. More honestly it's "we prefer having more time and money over having children" which is not even an objectively bad preference, but people don't like phrasing it that way because it sounds selfish.

So they say "we cannot afford it", suggesting "we _would_ have kids if we had more money", except in reality they still wouldn't, because at a higher income level they'd be making exactly the same argument, too. Which is why we see fertility rates _decreasing_ with income levels, up to a household income of approximately $500K/year in the US.