The inevitable fall of the euro?

There's been a lot of attention with the Euro lately, mainly because of Greece, and its caused a lot of economists to come out and renew their claims that ANY currency union without a fiscal Union is doomed to fail. Why is that? As John Cochrane points out in his blog here, a fiscal union doesn't have to be mandatory to get an efficient currency union. When Detroit declared bankruptcy, politicians in Washington didn't rush to decide if they should kick Detroit out of the dollar.
Is he right? Are the similarities between cities in the US the same as countries in Europe?
Some obvious differences are labor mobility and the political culture. Labor mobility in the sense that if any region in the US goes into an economic slump and people lose their jobs, they can move anywhere else in the US to find the same job or a different one. With Europe their tends to be a lot of frictions that could really deter mobility. An example being that if France goes into an economic slump, its workforce can't all move to Germany. After all doing something like that requires them to learn German, and adapt to their culture all of which increases the moving costs that it deters or at the very least slows down their movement.
Does that mean its doomed to fail? I say probably not.
One reason is that the worry that labor would move less frequently can be not as big as some economists believe. The OECD published a report here that shows that mobility right now may be even stronger than the US.  In fairness the data shows greater mobility in the newly joined eastern countries, while the rest of the Eurozone shows the same little-to-no mobility it has for the past few years.
Second is that the single currency union wasn't created as an economic program to stimulate growth. It was a political and strategic project meant to put an end to Europe's countless wars with itself. No country would presumably want to go to war with another country that it shared a currency, and had strong trade ties with.
But that won't stop some economists from predicting the end of the single currency. In the end nobody knows for sure.