Earlier quoted context omitted.
> No one wants 125 million thoroughly non-Sinicized Japanese inside the country; that would be seen as an endless headache, not a prize. I don't think what you claim the people want matters (if even true). Look at Tibet and Xinjiang
Xinjiang and Tibet have been part of China for many periods throughout history; Japan never was. At most, Korea was merely part of the tributary system. There is a fundamental difference here.
Claims to the contrary are largely historical revisionism. (As are the various claims that Tibet was culturally influenced by China - the story of Princess Wencheng bringing agricultural technologies to uncultured Tibet, as it is often taught in Chinese schools and portrayed in period dramas, is a myth that only came to popularity during the Chinese Civil War.)
Remember also that until 1951, Tibet occupied Chinese territories more often than vice versa - although given the case of Manchuria, China might actually see this as an argument in favor of Tibet being Chinese.