If the PRC should actually decide to invade, it is going to be extremely difficult to hold that off on their own for an extended period of time. Which means they need allies who can rapidly deploy a sufficiently large force to stabilise the situation.
But the only way to get there is with a naval force, and air supremacy would likely be critical to the outcome of that fight, which means you need someone with a large carrier fleet, and that is pretty much a pool of one.
Without US help, there is very little hope that Taiwan would not be overrun sooner or later. Their only real hope would be a nuclear weapons programme that would allow them to credibly threaten to nuke Beijing if invaded. But the PRC would never let it get that far and would make sure to strike before that could be completed.