In most of east Asian history, only China really mattered. Korea and Japan didn't. Taiwan didn't even exist.East Asians are particularly divided as a cultural group, in this regard. The distance between Slavs and other Europeans is probably the same as between Chinese and Koreans / Japanese, but the latter two groups are vehemently opposed to being associated with Chinese and would rather go down in flames with the West.
We shall not even mention the Taiwanese.
Yes much of this is also due to bad blood from World War 2 & divide & conquer tactics from the West. But the fact is there is almost zero feelings of “common identity” between East Asian nations that’d require the West to fear a unified East Asia. The idea of Japan or South Korea ever threatening to leave the US sphere for China’s based on shared heritage is so absurd that US strategists basically take them for granted, it’s comically sad to see.
Of course they would want to side with the US. They don't want to go back to being nobodies like before.
But that's not China's problem. In the end of they lose, they will bow down to China just like they do to the US.