The problem with crashing fertility rates is related to Say's Law[.
(interactive graph at link)
At least two children per woman—that’s what’s needed to ensure a stable population from generation to generation. In the 1960s, the fertility rate was five live births per woman. By 2017 it had fallen to 2.43, close to that critical threshold.
Population growth is vital for the world economy. It means more workers to build homes and produce goods, more consumers to buy things and spark innovation, and more citizens to pay taxes and attract trade. While the world is expected to add more than 3 billion people by 2100, according to the United Nations, that’ll likely be the high point. Falling fertility rates and aging populations will mean serious challenges that will be felt more acutely in some places than others.
While the global average fertility rate was still above the rate of replacement—technically 2.1 children per woman—in 2017, about half of all countries had already fallen below it, up from 1 in 20 just half a century ago....