It's literally an island close off the coast of China, like Galveston, the Florida Keys, or Long Island. It's literally inhabited by Chinese people. It's literally been part of Chinese territory for centuries. Just like Hong Kong, if you bring an enemy military into the region from the other side of the world and they enforce a separation of the land from it's historical roots, that doesn't make it legitimate.

Eventually Alaska will need to go back to the people who originally lived there. The European colonies that became new nations do not have legitimate claim to those lands. Just because they killled everyone who disagreed with them and then filled the land with people who agreed with them doesn't mean the claim is legitimate.

Similarly, just because the KMT killed everyone who disagreed with them and brutally indoctrinated the entire island through the White Terror and just because the imperialist Europeans provided support and military protection while it happened so that the Europeans could still plunder Asia, doesn't mean that the claim is legitimate.

Reintegration is the only sustainable outcome and China will wait for as long as it takes, knowing that once the US empire declines far enough, Taiwan will have less and less incentive to remain belligerent and will eventually vote for a one country, two systems arrangement.