I'm not convinced the economic argument is correct. Anecdotally, two is by far the most common number of kids among other parents I know. They're all pretty well off so I don't think that's the reason. It's definitely not COVID as their kids are some years too old for that. Nor do I think it's age or infertility, as they had plenty of time and there's no wistful talk about wishing they could have another one. As far as I can tell, they stopped at 2 because they wanted to.