China is a closed society dictatorship. They’re not attractive to higher educated talented people who have a choice where to immigrate to. It would be good for society as a whole for China to get smaller because of they get too big they will try to conquer a few other countries.
Japan is different and has changed over the last half century. They don’t arbitrary detain you or try to be repressive. Society will not lose out if there are more Japanese in the world.
In fact Japan does well with soft power, all my friends love holidaying there and some would even immigrate if the rules were relaxed. Whereas none would go live permanently in china even if they gave out PR on arrival.