As a fond former resident of Taiwan, I agree with the idea that there should be one China. But there should also be one Taiwan. Look at the map, and you will clearly see that there is one of each. A “One China, One Taiwan” policy would acknowledge the existence of both entities and also echo China President Xi Jinping’s “One Belt, One Road” slogan.
Theologians will argue that if God wanted China and Taiwan to be one entity, he wouldn’t have created the Taiwan Strait. Jesus declared, “What God hath joined together, let no man put asunder.” (Mark 10:9.) The reverse should be equally true: What God hath put asunder, let no man join together. Theology aside, if inhabitants of an island and its nearest adjacent land mass both want to be one political entity, that’s fine – there's no reason why they shouldn’t be. A problem arises only if they disagree.
There is a perfectly sensible solution that China refuses to accept, but that harmonises with all democratic and civilised norms. Let the 23 million people of Taiwan hold a referendum and decide their own destiny: whether to become part of China, or to be an independent, sovereign nation. Anything wrong with that?