China and Japan are too populous for immigration to be a viable solution.
Also, East Asian societies are generally too nationalistic and xenophobic to accept immigrants on a large scale. You see what they even quarrel among themselves? Chinese New Year or Lunar New Year, Dragon Boat festival origin, whose traditional clothing copy from who, whose cuisine copy from who, whose traditional culture copy from who, etc and all that nonsense. That's not really a great example of an inclusive society.
If even among similar looking people and similar culture they can feel so proud of their slight distinctness and feel the need to assert a subtle sense of superiority whenever such topic arises, you think they can really accept brown people as part of their mainstream society? They may ameliorate their demographic challenges with immigration but create more social problems down the road.