What’s wrong with their demographics? Population decline?

It has been called the 4-2-1 problem. 4 people had 2 kids. Those 2 kids had 1 of their own. This means there ends up with a more elderly people with far fewer young to support them. That doesn't look like a recipe for social stability. This is why they are going in so hard on automation nowadays, they are trying to do what Japan attempted in the 90s/2000s but hopefully with more success.

This was originally a side effect of the One child policy, but now it is continued due to difficult living situations. This is not a uniquely China issue.

https://alexatsintolas.weebly.com/the-4-2-1-problem.html