This is a big topic but as far as Demographics go, China is by far the worst placed big nation due to their one child policy and inability to support more than

This is a big topic but as far as Demographics go, China is by far the worst placed big nation due to their one child policy and inability to support more than one child due to modern costs. If that were the only factor or even a major factor, would be massively negative for China's future vis-à-vis rest of the world.