I think US would rather devalue the dollar by 30% in a very short amount of time than get in trouble over its debt. That would still be a catastrophic outcome, but I'm just saying that's most likely the option they will take.
Why default when you control the currency that all your foreign debt is denominated in? The us has an option other countries in this position do not.
On one side of the coin, people see that investment in US debt isn't so safe as they thought. Thus, borrowing becomes more expensive for the USA.
On the other side of that coin, America will have mostly escaped the really bad consequences of its behavior. I submit that we will not learn our lesson from it, and won't alter our behavior in any meaningful way.
I see those two things combining in a really vicious cycle.