[JasonC]

It has about $400 billion in foreign exchange reserves, mostly dollars but some Euros and some Yen. And last I checked, there were $4 trillion in US debts outstanding, which has another zero on it. Most of them are held by banks and other financial intermediaries, which means most of the final claimants are actually bank depositers. Not governments. Banks have depreciating liabilities to match their depreciating assets, so why would they care? Depositers deposit in no interest checking accounts and 1-3% CDs. Nobody shoots them if they don't, they could withdraw all their deposits tomorrow and walk around with pockets bulging with gold coins. But they don't.

[AdamSelene235]

This sums up the problem nicely:

The US as a Nebt Debtor: The Sustainability of US External Imbalances