Yeah..... The really bad part is that it would be most harsh on countries with unstable currencies. The USD is basically the foreign currency reserve that backs most poorer nations currency.

Countries could slowly start exiting the US bond market, but they would have to find a replacement, and nothing has been as stable or financially open to scrutiny as the USD.

Even if more countries switched over the yuan it'd have to be a slow process. There would also be a lot more blind trust involved since there's a lot of things done behind the curtain between the national bank and local government banks.