"Japan is particularly vulnerable to sea interdiction." Japan isn't the only nation with that level of vulnerability - so is Australia and New Zealand and all for the same reasons. Energy i.e., oil, war materials and food. Neither of our nations are self sufficient in any of those. 50 odd years ago we could quite easily feed ourselves, but now I think that is somewhat doubtful because of globalisation and our dependence upon foreign sources.
I don’t think Australia or New Zealand are as vulnerable as Japan and S Korea to sea interdiction. Still, it is certainly not desirable. Mahan stated that a nation’s economic access to the sea is the basic rationale of naval power. This includes fishing rights. But surely Au and NZ issues are more around another point Mahan made: the borders of a great maritime power end at the coastline of lesser nations.
We need to step back and take a look at the big picture. China harbours ambitions of great power status. But to achieve this, they must first become hegemon in Asia. Russia, Japan and India are their true rivals and must be convinced, one way or the other, to accede to the Middle Kingdom’s authority. China needs to evict the Americans in order to do this. A successful interdiction of Japan and other E Asian littoral states, will demonstrate China’s dominance and end America’s credibility as guarantor of regional security.
In time, sea interdiction, or even direct attack, on NZ and Au might possibly become a real concern. And, being the odd sort of fellow that I am, I have been pondering what might be done sans USA. However, preventing such dire circumstances would surely be preferable, hence my concern with thawting a Chinese interdiction of E Asian littoral states and there by sustaining US credibility as a counter balance to Chinese ambition.