You say that is if it's going to be easy to "whack them".

I didn't mean to give that impression. It will get ugly. It won't be a land war (to our benefit). China would have great difficulty landing troops in Taiwan as long as the US Navy is present. Plus Australia, possibly Japan, possibly Britain. There is an alliance gathering against China.

They could always launch missiles at Taiwan, which could then reciprocate. But occupying with ground forces will be really tough for them. Similar to how it was difficult for the Germans to invade Britain proper. The British (and allied) navy(ies) were an impediment that would have inflicted unacceptable losses. So it never happened.

Beyond that, war is relatively unpredictable. If the allies don't roll-over, they will prevail. It's a question of how much China is willing to spend. They would lose a lot of their export business because most western nations would impose a ban on Chinese products. That would probably be the determining factor as to how long China would last in a conflict.