Either that, or we would just fail to make the payments. That would crash the value of all foreign debts, and so the holders would be lucky to get ten cents on the dollar by selling their paper to speculators (who are betting that we'll pull out of it).
Failing to make payments on debt is a default. Perhaps you are not familiar with what happens to people or companies or even countries that are in default. People in default over their home mortgages lose their houses. Companies in default over their debt end up in bankruptcy and shareholders often end up with little or nothing. Countries in default have the value of their currencies (which is effectively a country's share price) drawn through the dust.
If the US defaults on all foreign debt, the value of the US dollar will be crap. Nobody will want to hold US dollars because it represents a liability that the country has shown it is not willing to pay back.
All US citizens would suffer the effects of hyperinflation as the price of all imports in US dollars soars.
Careful what you wish for.