This is summarizing a serious problem to its absolute essence. What is the biggest source of China's power. Its large population of one single ethnic group composed of 1.3 billion people unified under one single country, single culture, centralized government with absolute power.Bad is bad, good is good. This is not a very rigorous analysis bud.
This large mass is what gives China the power to compete with the collective west. The fact that China's population is larger than the whole west combined is what gives it importance. If China had all these characteristics but only had the population the size of Japan for example, then it would be nothing. It would have the importance of Iran for example, an annoying nuisance but not of central importance for the whole world.
China needs to keep this level of population to be able to gain and maintain its superpower status. Without it, no matter how rich or how advanced it becomes, it will not have global impact.
So, China keeping its population size is absolutely critical to the global balance of power.
Chinese Government will slowly realize this as well and will not be able to just stand idle while China's population keeps declining and their collective power slowly reduces. They will have to take action. Just like growing military power is a national duty for the government, maintaining and growing population size will be of absolute importance as well.