When we say backward or advanced economy, we say this by comparing it with other countries. For example, India is a backward economy compared to China or western countries, but it can be considered developed compared to some African countries. Or even the most advanced countries of the world are probably lagging behind compared to aliens. There is no measure of development in itself for me. Therefore, if China cannot maintain its leadership compared to other countries in the future, it will lose its developed country status again. In my opinion, in order to maintain the pace of innovation, the ratio of the young population and the population are important factors in addition to skills.
As to your question what the population should be. It is obvious that the current population is already sufficient, there is no need to increase any more. Even a slight decline will not be significant, but the current rate of decline should at least be slowed down and spread over a long time.
I am writing this not to reason about to make negative predictions about the future, but to discuss what the most correct government policy should be.