I understand this. My point was, almost every previous generation lived with objectively less financial security and a lower standard of living, and they still chose to have more children. We can't say people are more financially insecure than before. It just isn't true, on the whole. So, logically speaking, it must be true that those same concerns would often not have stopped this hypothetical couple's grandparents or great grandparents... Because they didn't. And I don't think you can say in good faith that they all had more children for monetary reasons like labor.
Look at first generation immigrants from third world countries. They tend to have a ton more kids than the existing populace, while at the same time being much poorer than average. Then, by the 2nd (or 3rd) generation, that birthrate difference disappears completely. I bet if you were to ask this group why, they would talk about financial insecurity. But that didn't stop their parents (or grandparents), despite being much more financially insecure.
So besides being more well off, what else changes between 1st and 3rd generation immigrants? Integration into the Western culture.