From a macroprudential perspective, the real issue concerns not so much that the US government is indebted, but the fact that its capacity to service its debt falls as production (and by extension, tax revenues) is outpaced by debt. My issue with all of this is not the apocalyptic scenario of the US becoming Greece 2.0. It's the considerably likelier scenario that one inconvenient structural change pushes interest rates up for a decade. You wouldn't need a very large swing for hikes in debt services to force the hands of Congress to take rash, very likely idiotic large scale action to redress its finances. The US government gets incredibly cheap "loans" by issuing fixed income instruments periodically, but suppose we enter a period where interest rates start rising back and the new normal is not one where the Feds Funds rate is basically pinned down to 0-2%, but more 4-5%... The yields on those bonds would follow suit, their prices would fall and, soon enough, those debt service payments would become a very serious problem, eating through the budget every year.
That part, I think, is a political, social and economic threat. At the economics level, rapid changes that take everyone by surprise aren't exactly the best way to handle changes. Ideally, you want people anticipating what will happen and move carefully toward new regulations, programs, reforms, etc. The last thing you want is people waking up one morning and learning large scale transfers of wealth just happened because program X was cut in a haste overnight. Regardless of what you think about the morality of program X, sudden cuts that take everyone by surprise might put a lot of people in a tough spot where they take surprisingly similar decisions all at once. Any problem in that kind of environment would not be isolated and personal. At the social and political level, having Congress being pushed against the wall by creditors is not exactly how you incentivize politicians to make choices aligned with the public interest. If you think they're bad now, imagine if any one group of people literally has all of them by the balls.
