That’s why the US is stagnating and China is growing at an incredible pace. In 30 years they created a massive middle class.
China has been a great success story in many, many ways, but they have now passed the point of "easy gains." Central planning, as with all things, works--until it doesn't. In retrospect, it seems like they held onto the one-child policy for too long.
China's population is massive and has historically been massive (relative to the rest of the world). When the Founders were signing the Declaration of Independence in Philadelphia (population circa 30,000) and the 13 British colonies had a population between 1.5 and 2 million, China's population was probably over 300 million. Anything that happens in China is massive!
China should be commended for their efforts to grow the middle class, though I feel the evidence is not entirely there yet. China's population percentage that's middle class is still dramatically smaller than the EU, US, Japan, etc. Potentially even Russia. (The Soviet Union is another great success story for pulling an absolutely massive people out of literal serfdom and into the middle class.)
Today, China's total fertility rate is reported as one of the lowest in the entire world--0.93 in 2025. And that's if you believe the reported numbers. East Asia is being hit very, very hard by crashing reproductive rates. It's happening all over the world, but China, South Korea, Singapore, Thailand, etc., have just crashed.
Stagnating can mean many things (population growth, cultural growth, scientific growth) and I'm talking specifically population as the rest is downstream of that.