The largest holder of US debt is Social Security fund so SS goes bye-bye forever.
The second largest holder of US debt is pension funds so 90%+ of those go bankrupt within a few years.
Insurance companies (oop your life insurance policy is now worthless) are another major holder as are local/state governments (oops states go bankrupt too).
What do you think that will do to retirees all over the globe and the spill over into larger economy?
When every retiree loses a massive amount of lifetime disposable income forever what do you think that will do to global economic demand for goods & services? When those companies see demand fall 20%-30% what do you think they will do to their exist workforce?
When 500 million retirees all over the world are forced to work again to avoid starvation what do you think that massive influx of labor will do to global unemployment (already suffering from unprecedented layoffs).
Now that is just one aspect of that massive economic shockwave. Now the govt will step in to save the day right? How are they going to do that without borrowing? Print money. Massive global inflation destroys any remaining wealth American have.
Quick look at 17 largest holders of US govt debt:
http://www.cnbc.com/id/29880401/The_Biggest_Holders_of_US_Government_Debt