I am not so sure that even if the US economy goes to hell in a handbasket, it will mean that much for the dollar.
2/3 of the dollars in circulation trade outside of the USA.
The dollar is a primary currency among peoples in Uzbekestan and the Congo and many other places in the world who will never go to the US, couldnt tell you what the current interest rate on a mortgage is, and dont care.
A hundred dollar bill, north of Chang Mai, will still buy you almost anything. Doesnt matter who is president, or whether Lehmann Brothers exist.
So my guess is that the dollar will remain a trading currency for some time, even as US belts tighten and taxes go up.
There is no credible threat of the US government defaulting on treasury bonds, or in other ways affecting the value of the dollar as a trading currency in other countries.