Because every sophisticated technology on planet earth requires their 10 nanometer chip manufacturing plants. China can make worthless chips used in industrial controls or lousy cars, but only Taiwan can make cutting edge chips.
A world where all foundational technology is under Chinese rule is a game-over.
Add to that, with Taiwan under Chinese control, there is no reason for the USA to have presence within their waters. That sees us tucking tail to Japan or the Philippines, leaving the entire region susceptible to further Chinese aggression and hegemony.