All economics is about making choices among scarce resources. In most of the world over the past century, people have been in abundance. Now they are increasingly scarce in most affluent societies. The average woman in Singapore has only 1.5 children, compared with the 2.1 needed to achieve a stable level of population. Taiwan and South Korea, too, are close to the European norm.
Japan already has one of the world's oldest populations.
That is even more the case in advanced societies than in developing ones. However much money we save today, our standard of living in retirement will be primarily determined by the productivity of our individual national economies. And that is directly related to availability of the human factor of production. There is, of course, much more to having children than just brutal economics. But let us not imagine that economics doesn't enter frequently into government consideration of family size and birth rates.