to answer the original question, as if that will ever happen...
world war 3.
no seriously, if we ever look at the rest of the world and say "oops, we can't pay you back now. sorry". it will start a world war. multiple countries will gang up and invade us.
look at the scale. no matter how big or small money has always started some of the largest and ugliest feuds. the U.S. looking at the rest of the world and saying"tough ****, get over it." would be the biggest slap in the face ever heard round the world.
the brighter your light the darker your shadow.