Its unlikely that a single currency (such as the Euro, Yen or Yuan) will ever replace the $US as the worlds default trading currency. That would simply be inviting another disaster down the track such as has been caused by this experiment of the past 38 years with the $US hegemony. A far more likely scenario is that a basket of major currencies with perhaps some precious metals thrown in would become the measure of value for individual currencies. And it is almost certain that the $US would be a significant component of that new world trading currency.

I feel sure the groundwork for such a change has already been laid by governments around the world as it is in no ones interest to see the $US completely collapse.