On an abstract level it does seem better to have a large number of young people to care for a small number of retirees, but that would create an unsustainable pyramid scheme in a country like China. Many parts of the world are already overpopulated and low fertility is pretty much the best case scenario.

All of that aside, I also think that there are factors more important than the relative number of old people in a given population. Technological development, public health, etc. are all probably more important, and some of those factors are at odds with high fertility.