The USD is a fiat currency that is worth what people (the market) values it at. The ability of the US government to pay it's debts is but one (very important) factor in that distributed calculation. It is not solely determined by that. If it were the value would rise or fall solely on the debt-to-GDP ratio - which is probably only loosely correlated - up to a point where default becomes probable and then it matters an awful lot more, like with Argentina.