China could replace all the non-functional Japanese men rather quickly.
True, but it would make more sense for China, with its surplus of single, young men, to hit Korea or Taiwan first. Also, China is expanding into Siberia, which is contiguous, like Korea.
Ten to one advantage in population, and the Chinese aborted millions of baby girls, causing a terrible imbalance of men to women. And the Chinese have a score to settle over, let's say, among other things, the Rape of Nanking. The Japanese probably need to be looking to acquire some nuclear weapons. Or just surrender when the Chinese figure the above out.
Especially given that China has a shortage of women because of their one child policy.