I agree. It comes down to the opportunity cost for women to have babies.
On pre-industrialized societies, women have barely a choice. On industrialized ones they do. And it turns out that, when given the choice, they choose not to have babies.
The implication of "and it turns out..." is that all else is equal, but clearly it's not. Would women still choose to have babies if they didn't have to work also? I admit that it's basically moot - we can't seem to figure out how to have a society where both members of a couple are free to choose whether or not to work. I'm only pointing out that this trend doesn't mean what you're implying about women's desires.
If you don't think child rearing is work then you won't understand why women choose not to have kids in the first place. You cannot be a parent and choose not to work, period. Just because you're not getting paid and ordered around by an adult boss doesn't mean being a trad wife is magically somehow not work. In fact, at least with a regular 9-5 you get PTO and time off.
If you scoff at the idea of flipping burgers your whole life then just imagine it's changing diapers instead.
Would it make sense to frame this as a Baumol's Cost Disease problem? E.g., the labor of child rearing has been historically offset by the inherent emotional surplus of the task, but the march of productivity in other sectors gradually increases that imputed loss until we reach a breaking point.
On pre-industrialized societies, women have barely a choice. On industrialized ones they do. And it turns out that, when given the choice, they choose not to have babies.