> I cannot imagine deciding to give up a child because of a minor inconvenience like this.

I don't think the article was suggesting that parents sit down together, ask each other "hey, should we have a third kid?" and one of them says, "well, I would say yes, but the only downside I can think of is that fitting a third child seat will be really hard, so let's not do it."

Parents who decide not to have the third kid certainly have other reasons for not wanting one, and the difficulty of a third car seat is a contributing factor, not the only consideration.