The US will also not pay back it's debt. This is not how it works. You do not pay off debt. You refinance debt with other debt. So this "There's no way Greece's debt can ever be repaid in full." is beside the point.
The US is an anomaly in that it has had the printing press for the world's reserve currency for more than forty years. Whether it'll be able to do so indefinitely is a different question.
Apropos this: