Well, obviously the US is not going to tell its creditors to go screw themselves. They'll just re-structure the debt in a less favorable for the creditor way. And the latter will agree to that because it has no better options.
That will still tank the economy.. creditors no longer have liquid funds and trust is gone. Any future bond issues will have very interest rates, while the debt now is not actually very large compared to gdp in the U.S and is quite serviceable, if the interest rates become high it will become difficult to service it causing runaway effects and future investments/spending will be hampered leading to all sorts of problems
