All that really matters is if a nation can sell its debt and pay next to nothing in interest. A few years ago Germany was selling bonds at a negative interest rates and nations were buying them. They were safe. Greece was offering 10-15% returns and no one would touch them.
The US is never going to pay down any debt. All that matters is if they can service the interest. As long as GDP keeps growing, its not really an issue. Japan has twice the debt to GDP ratio of the US.