But they really shouldn't be concerned. Fisk's theory would make a good plot for a Hollywood movie, but it doesn't make much sense as economics. It is true that oil is priced in dollars and that most oil is traded in dollars, but these facts make relatively little difference for the status of the dollar as an international currency or the economic well-being of the United States ...
I do not want to be the devil advocate but basically US is paying oil in paper as the US print dollars. Changing the currency on oil market from $ to euro will allow Europe to pay oil in paper, just as the US.
Then the US will have to:
1) pay in foreign currency that they will have to purchase on international market. So the price variations of oil will impact directly the economy of US as even oil produced domestically will be subject to a world price in foreign currency.
2) China is one of the biggest owner of US debt. Actually it does not worst even 1$/$. As curious it can be the bonds have a lower value than the one printed on it. A change in international currency for oil would impact the value of the $ on the market exchange. Then China (for example) will ask to be payed back for all the debt they own. What is actually quoted 0.9$/1$ may in deed be quoted 1.2$/1$.
Just take the time to make the calculation of what represent 50% of US debt but with a 1,2$ value of each $ of debt.