> "The economic argument would be that some of the benefits of creating new people are captured by the rest of society, not just the parents, and so to maximize efficiency, some of the costs should also be borne by society."

If you replace "new people" with "works of art" and "parents" by "artists" you have logic that applies equally to a universal subsidy for all artistic activities. This might be nice, but it's something that most people would recognize as impractical and idealistic. But when children enter the picture people lose all rationality and revert to a need-focused mindset instead of a resource-focused mindset.

An actual economic argument would be much simpler than the one your propose. The production of children must be subsidized if and only if it is an objectively valuable and necessary activity (which it is) but insufficient people are dedicated to the task (which is plainly not the case, as evidenced by the stable population of the US).

Reasoning based on who enjoys the benefits of an activity is not economics, it's philosophy.