Well for one obvious example is both of those societies are only recently atheist (<100 years) and that they both have an absolute dismal birth rate that makes it hard to imagine how they will look like a hundred years from now.
Buddhism is an atheist philosophy. It has been around longer than Christianity. It has hundreds of millions of adherents.
Do you disregard it because you don't think they are successful, or numerous enough?
Every society has had gods at some point in the past. Why didn't every society improve then? And China's growth has been very recent(after the 80s). And birth rates have absolutely collapsed across the world. It's not some unique Chinese/Japanese situation. Before their silly 1 child policy change China had insane population growth. As non religious people. Almost like these aren't really connected at all.