They cannot defend the Taiwan strait against our navy. They know this. If they attack Taiwan they mean to destroy it.
Given that a sizeable chunk of the US electorate is wearying of the commitment to Iraq (already), I think the US defense commitment to Taiwan is in serious question.
It seems pretty clear that China is manipulating the North Korean crisis to their advantage. Any move against Taiwan will probably come after a military confrontation on the Korean Penninsula.
How does our Navy defend against an attack that consists of a call from Peking to all COSCO ships, ordering them to turn around?
I give us 30 days of non-supply from China before we're on our knees.
We've exported our manufacturing infrastructure. We have placed ourselves at the mercy of our avowed foe. We have made ourselves dependent on an enemy for our very survival.
We are fools, and China has been laughing all the way to the bank.
So, what will our Navy do? Mount an antiblockade?
Hint: There ain't no such thing.