Switch allies, to whom? The PRC? Do you realize PRC (Mainland China) and Taiwan are in a civil war? Taiwan switching allies to the PRC literally means admitting defeat.

Recall that Taiwan only exists because during the Chinese civil war, the United States provided the KMT with billions of dollars of military and financial aid. The KMT lost the mainland to the CCP (supported by the Soviets) and fled to Taiwan. The U.S. provided $4 billion dollars of aid, worth approximately $50 billion today.

The PRC would have retaken Taiwan decades ago if not for the U.S. guaranteeing Taiwan.