Population alone does not guarantee success, but in China's case, its population was key to its success, and losing it will not benefit them.
China's population played a role, but probably much less of one than is commonly thought. Most of the industrialization in China was on the coasts and the productivity of the population in the country's interior was much lower. Right now, there's been more of a push to get more out of the interior but it still lags behind by a lot. And in the long run, it's going to be the ability to increase productivity that's going to matter more than the total number of people in the workforce. Another way of putting it is that a factory that is more efficient but employs fewer people is going to be more useful than one that employs more people but is less efficient.
Also, it should be noted that access to lots of cheap labor was key to China's development from the 1990s to 2010s, but that was because the country was mostly doing labor intensive manufacturing lower in the value chain. China has moved up the value chain so this brute force approach isn't that important any more.