The dollar is much like capitalism and democracy: it's the worst imaginable choice, except for everything else that's ever been tried. That's really it: the US government is just less effective at ruining the dollar than every other central bank for a major currency. Much of the current value of the euro is simply it's position as "second lest awful currency", a hedge against a reckless Fed.
When the dollar looked rough a few years back, interest in gold perked up, and interest in the Swiss franc bloomed, but the Swiss were apparently uncomfortable with the idea of becoming a reserve currency, and committed explicitly to falling with the euro. Odd choice, but there it was.
If the EU ever finishes collapsing, all the sovereign debt in the region blows up taking the Euro with it (it's only a question of when), and then finally recovers and regains financial credibility, then finally the dollar might be displaced, Or if China has it's inevitable revolution, and happens to emerge as a capitalist democracy, or when India or Brazil finally reach first-world status nation-wide. we might also have other credible currencies. But for now, the dollar is pretty much it.
That's not to say the dollar can't be destroyed if the Fed tries hard enough. But over the past downturn they were actually quite clever: while they were printing ~$2 T in no money via QE, they were removing ~$2T in money supply via bank reserves (while we continue with a 0 reserve banking system, the Fed paid attractive-enough interest that banks voluntarily increased their reserves to unprecedented levels). And the Fed did actually wind down QE. I'm still waiting for the other shoe to drop, when that ~$2T in "Excess reserves" comes back into the money supply and turbo-charges inflation, but hopefully that will just be Carter-like, and not a currency collapse.
