What One China policy did the US sign? China has a One China Principal which some countries uphold, but the US is not one of them. Our _policy_ is to acknowledge China’s position but does not endorse or challenge it. Chinese diplomats are eternally trying to conflate the two, saying even the US admits there is but one China, and thus supports China’s position on the Taiwan question, but that is not true.
As for the silicon shield, yes it’s probably a major factor, but not close to the only factor. If PRC realizes its claims over Taiwan and the South China Sea, the entire region, including many US allies, will be under Chinese hegemony. It would spell the end of the current economic order. Japan, Philippines, basically all of East and Southeast Asia would then be trading within China’s new backyard.