Since the United Nations is supposed to provide a voice for all nations, I see no basis other than China's objections as to why Taiwan is locked out of the U.N. I also think it is time for the U.S. to recognize Taiwan as independent and self-governing nation.
Over the years, American foreign policy has taken several unrealistic approaches in regard to China and Taiwan. For more than 20 years following the communist revolution in China, we had a "one China" policy pretending that a nationalist exile regime based in Taiwan was the legitimate government of the Chinese mainland. In the early 70's, the U.S. adopted the asurdity of a "Two China" policy and then soon shifted to another "One China" approach favoring the mainland Chinese government.
The U.S. should recognize that while there is only one China that Taiwan is a soverign nation which has been separate from mainland China for over 50 years. Americans across the political spectrum should support the human rights of the people of Taiwan by supporting their continued independence despite the threats from mainland China.