In fantastical theory, if China destroys the entire island of Taiwan and its people in just a matter of minutes, is there a country that would avenge Taiwan?

Depends on the UN response.
What in a fantastical theory would China gain to wiping out Taiwan?

Yes. The civilized world would take powerful actions against China such that it would never recover fully. That’s why China hasn’t invaded and taken over Taiwan yet.

I just googled it and Taiwan is considered a non-NATO ally by the US, and the US would come to its aid in the event of an unwarranted attack.