The west doesn't need other nations to have high fertility rates, it just needs other nations to have people who want to immigrate to the west. Populations keep

The west doesn't need other nations to have high fertility rates, it just needs other nations to have people who want to immigrate to the west. Populations keep increasing for many years after the birthrate falls below replacement rate due to population momentum (people individually have fewer kids, but there are more people having kids than there used to be). And even after the population truly stops growing, people will still leave for wealthier areas in search of higher quality of life. If people keep emmigrating from a country with a sub replacement birth rate, eventually the population will get so low that there's no one left, but that would take many decades if not centuries. For example bangladesh has a net migration rate of -0.2%/yr and a birthrate 5% below replacement rate. Even if we ignore population momentum and assume bangladesh's population will immediately start falling, if they keep their current birth rate and migration rate it will take about 250 years for the population to halve, at which point it will still be about the same number of people that Germany has today.