Why do you all have this obsession with repaying debt? The US debt will never be repaid. Maturing debt will be replaced with newly issued debt. It will never be repaid and it doesn't matter.
As to how long the US$ will be the world's reserve currency, the answer is "until something better comes along". What do you think is a better currency than the US$? Certainly not gold because it can't be used for electronic transactions.
