Much of that is due to the capital flight toward safety (at one point, even turning US Treasuries into slightly negative-value investments). So long as people with wealth are willing to park it in US Dollars, you won't see any major shifts in the currency. However, provide a viable alternative and I think you'll begin to see some rapid shifts that won't make anyone holding assets denominated in US Dollars very happy. And that could trigger some truly catastrophic investor and sovereign panic as people and banks (including national banks/treasuries) start dumping those assets for anything and everything else to avoid being wiped out by a hyperinflated Dollar.
Obviously this requires some sort of replacement reserve currency. It can't happen until there's something else considered as or more safe. The Euro would have been the best contender had it not been for Europe's debt crisis and hard hits during the downturn. God help us all if precious metals ever fit the bill from large scale institutional investors' perspectives, because those are a known-workable entity. It's merely a matter of perception of safety.