That excellently sums up the reason why US and Japan are likely to intervene in a violent attempt at re-unification. They either do that or cede their dominance of the western Pacific to (from their POV) hostile China.
Having said that, I still think it is possible for China to achieve unification without a fight and in a way that would not be challenged by the US and Japan. Long ago, Sun Yat Sen had a dream of a unified, strong and independent, yet democratic China. If the CCP can make that dream come true, then Taiwan would be irresistibly drawn into its orbit and both the US and Japan disarmed in their attempts to resist it.
If they don't cede their dominance of western pacific in peace, then they will fight a war in China's own backyard, lose badly, get humiliated, lose huge number of ships, planes and people, and then lose their dominance of the entire world since losing all those hardware and troops will take a long time to replace and many players around the world will take that opportunity assert their power.
This is the choice China will force upon US after it gets stronger than US in the next 20-30 years. It will be double US GDP and much bigger military budget. Then even if US wants to fight China in its backyard, it will see a certain defeat and back down from it. Or it could become foolish and commit a blunder to actually fight and lose which will cause massive loss of US power all over the world.