1. The U.S. fails to protect Taiwan. 2. All allies lose confidence in the U.S. nuclear umbrella, and they create their own nuke programs. 3. Japan and S. Korea produce nukes nearly overnight. Lots of other countries follow. We now have a much worse nuclear arms race.
China needs only one reason to make the argument for taking control of Taiwan. That's national security. Taiwan is extremely important as a strategic island for defense of the mainland. For example, go look at a map of Taiwan airspace vs Chinese airspace. Honestly, I find it almost ridiculous that such a great power is boxed in so much by a tiny neighbor.
Personally, I think Taiwan needs to take one for the team and commit to reunification with China. That would cool things down significantly. Maybe the Chinese could even throw in some deals on borders to give the Philippines and other neighbors a break.