Japan only need to win once, and they almost did. You never know what happens after 500 years. Never wise to leave a large independent entity protected by ocean near the heartland.
They cannot even win once. They think they almost did but not really. When the mongols "won," China just absorbed them by numbers, and the mongols disappeared but China's still there. If you never know what happens in 500 years, then you never know how China will behave, whether it will continue to be merciful or if Japan can even sustain itself for that. The way things are going, when the US falls and Japan becomes more sterile in decline, China might reclaim them, giving a boost to both Chinese and Japanese populations (Japan province reinvigorated by Chinese vigor and the rest of China refreshed by embracing Japanese culture and dedication to craft). And remember, just as Japanese cruelty comes from innate frailty and insecurity, Chinese mercy comes from strength; it is not the Western sense of weakness-induced hesitation. If China's strength overmatch fades against Japan, so does our sense of mercy.