This misses a bigger issue with a baby bust: population composition. When birth rates decline sharply all at once, as is happening here, populations skew older and older. More elderly people being supported by fewer young people is also not a recipe for sustainability, in fact it is quite the opposite. The people who will be burdened with caring for the elderly are also the people who will need to have children in order to stabilize the population. This isn't an easy leveling-off of a population, its the beginning of a collapse.