For me it has a possibility that USD can be used up for illegal activities. Because such as terrorist create and use this money on anything they wanted. Unless, someone can detect and stop this bad activity.
In fact all currency in the world was used in different criminal activities, maybe USD is included in this some illegal transactions. Even bitcoin have a risks and there's a possibility that in the future, bitcoin value was used by the terrorist to buy some items that they used in war. Because if we analyze all terrorist are a lot of connection in different part of the world.