.... Yes. I mean, if you want to do a phd's worth of work, there are existing datasets one could mine - but the time horizon (since the legalization of birth control) is so short and the social context regarding reproduction has been shifting so heavily during this period that any predictions you make would end up being barely guesses. Fortunately, the subset of plausible futures in which this matters is absurdly small. The world would essentially have to enter into technological and social stasis for many thousands of years, and well. Uhm. No.

The marching morons has a lot to answer for, really, since variations on this is an idea that crops up like weed, and it is a pretty absurd scenario.