Edit: Since this was down voted, I want to be clear: this is not snark. This was meant as a very real commentary on how important it is that we have the ability to inflate our way out of debt. It's critical to understanding the power dynamics at play here.
There is no need to pay off the debt, as long as it grows slower than the economy it is shrinking in real terms.
Furthermore it serves an important purpose as a "risk-free" investment of US dollars and as a tool for monetary policy through open market operations. In fact there isn't enough outstanding debt for the second role, which is why during the financial crisis the Fed expanded its balance sheet to include mortgage backed securities.
Finally any country with its own currency can't really default on payments, except in a technical sense like the "debt ceiling". Instead if the government continues to run expanding deficits then inflation will rise and smooth things out, statistically reducing the debt. Think of it as everyone who owns dollars pays off a bit of their share of the debt.
Severe inflation like in the 1970's or worse is clearly harmful, but deflation is more harmful than moderate inflation. We have been running below policy maker goals of about 2% inflation for a while, despite the cries from deficit hawks that massive inflation is right around the corner due to QE.
As of September 2014, foreigners owned $6.06 trillion of U.S. debt, or approximately 47% of the debt held by the public of $12.8 trillion and 34% of the total debt of $17.8 trillion.[42] The largest holders were China, Japan, Belgium, the Caribbean banking centers, and oil exporters.
https://en.wikipedia.org/wiki/National_debt_of_the_United_St...