Solving population collapse with immigrants doesn't really work in the way many seem to think. In this case, your examples demonstrate one of the many issues to

Solving population collapse with immigrants doesn't really work in the way many seem to think. In this case, your examples demonstrate one of the many issues to a quite extreme degree. The entire population of Yakutia is 1 million, Mongolia's about 3 million. Since 2010 Japan has lost more than 5 million people, and their rate of decline continues to accelerate. Even putting every single Mongolian and Yakut into Japan would fail to replace what they've lost just recently, let alone create a sustainable population. Beyond this there are also many other issues. Neither of these countries speaks Japanese. And while I don't know much about Yakutia, Mongolia has an extremely high crime rate for Asia. And I think that replacing the Japanese with large number of migrants would almost certainly just destroy everything that we all want to preserve about Japan - it definitely has the most peculiar and distinct culture among developed nations. Migrants will bring their own culture, values, and ideas. That can be a good thing ('Japanese' tempura is largely a product of the Portuguese for example) , but at an excessively large scale it can also simply drown out instead of compliment the host culture.