“The USA is monetary sovereign with fiat floating exchange rates.by definition it can never run out of USD.”
True. But the open question is what value the rest of the world places on a unit of USD, and how that might change with time.
Nobody knows the answer to that. What we do know is that (a) the ratio between receipts and debt continues to erode, and that cannot be assumed to be anything other than a headwind for value, and (b) nothing stays the same over time.