Here are some very basic statistics, to give you an idea of why we are on a journey towards too few children, below the replacement rate of 2.1 per woman:
Globally the fertility rate has dropped from 5 to 2.4 (children per woman) between 1970 and 2016.
In high income countries it is already below replacement rate, on average, dropping from 3 to 1.7 in the same period of time.
China fell from 5.7 to 1.6. India went from 5.9 to 2.3.
Sub-Saharan Africa, dropped from 6.6 to 4.8.
How quickly Africa catches up will depend on how quickly we see female-empowerment, access to birth control, and reductions in poverty. These are big unknowns, and the outcome will help determine how quickly we have global population decline.
Source of data: World Bank