Earlier quoted context omitted.
How come? Now, they just sell more I.O.U.s to pay the ones that reach the due date. They could do exactly the same in BTC, gold, tulips, or whatever.
No one distinguishes between "original" USDs and accounting IOUs in a bank. In Bitcoin, people can and do distinguish between a real Bitcoin and IOUs for one.
Sure, they're traded at very close to face value, but that's because traders trust the US government to pay. That's different for governments considered at risk of default. That could also become different for the US government, if the economy really goes south or if enough people start talking about defaults.
I don't see how criptocurrencies would change any of that. For how much less than 1 BTC would people trade a 1 BTC IOU backed by the US government?