Japanese sunk immigrants

If Japan sinks, the Japanese have few choices, because it has too many people, like Canada; Russia; United States; Brazil; Australia, with a small population, will be very wary of the entry of a large number of immigrants. Japan can only be fragmented. Although China has a large population, it has a vast territory, and China is big enough with similar culture, so it should be a good choice. If the Japanese are smart enough, they can change themselves and integrate into China, just like the Xianbei people in the northern part of China. But if Japan's current national conditions do not change, there will be no such possibility.