Numerous polls of the Taiwanese show that they would want to join the US if given the choice between independent statehood, joining the mainland, and joining the US.
Plus, the US gets a nice permanent military base and gets to monitor all future Chinese submarine activity.
If China attacks a US state, it would be like Yamamoto's "sleeping giant" moment. The US is trying to force China's hand while there is power asymmetry, and I can't think of a better checkmate move.
I mean, imagine the scenario where Taiwan becomes a U.S. state and then China invades and takes it anyways. And suppose the U.S. can't take it back without all-out (possibly nuclear) war, so they back down and let China have it.
That's a possible outcome that would have to be contemplated if Taiwan were to join the U.S.
China only loses its oil shipments if it can't secure shipping through the Indian ocean or build an overland alternative. It won't make a move until those are complete.