When most of society was agrarian and labor-oriented, more people meant more hands to do the heavy hand-work of maintaining the city.
Modern automation and standardization has largely removed those benefits; a single person can operate a tractor that maintains acres of crops on an almost semi-autonomous basis, with the driver there mostly to handle breakdowns and deal with the unexpected. Having a family of eight kids means you've got one tractor driver and then what?
Parenting seems like an example of L.A. Paul's "vampire problem" - where you're such a different person after an event that it's not possible to evaluate it as the person you are before the event. But it's actually more complicated than that.
If I had to choose between being completely destitute and one of my kids' lives, it's not even a decision. But when considering if to have another child, comfort and affordability have a huge influence - even though, as a parent I already know that the moment the child is born, they would become incomprehensibly precious to me.
So I have the ability to bring into existence something would for certain massively increase the joy and purpose in my life, and yet I don't... for the same reasons that I criticized you for in a sibling comment.
This trait also naturally removes itself from the gene pool.
I think it's a lot more complicated than a simple genetic trait: people weigh the cost and benefits using a lot of no-single-gene high-level reasoning and (in this environment, the massive inequality in haves and have-nots, the relative lack of material support for raising a child from the larger community, the overall ease relative to decades past of access to the tools of controlling pregnancy, and the removal of most of the patriarchal obstacles to women in particular remaining marriage-free) decide that it's not worth the cost. It's not like there's a risk of failure to propagate the species; if anything, the risks seem most associated with increasing the headcount, not decreasing it.