They might do it because of the value in denying China access to Taiwan. Even assuming that Taiwan blows their semiconductor fabs to prevent China from getting

They might do it because of the value in denying China access to Taiwan. Even assuming that Taiwan blows their semiconductor fabs to prevent China from getting them, there's still all the know-how and all the other stuff that the US wouldn't want China potentially getting its hands on, like American weapons. There would also be the consideration of the risk to Japan (and other friendly nations in the region) created by allowing China to just take Taiwan without at least Ukraine levels of backlash.