Until they stop paying it. Consider that possibility, and it is not impossible for it to happen. Chi-Coms seeking to take over reserve currency and succeeding at least in some countries.
I am aware that the Chinese and Saudis are trying to supplant the Petro-dollar.
If they succeed the US dollar will suffer hyper inflation as all of those off shore dollars comeback to the States.
A world wide depression is also a very likely result.