I think China's waiting until the US is so broke it can't raise the money to rescue Taiwan. Based on the 40T debt, 2T deficit (not counting the 1.3T mid-term election bribe) and recent bond market jitters, that could be quite soon.

The problem China has is that as long as they depend on exports, they depend on the US consumer. If the US consumer stops spending, there is nobody else to pick up the slack.