For a single-currency area to be sustainable, one of two conditions needs to be met. One, sufficient economic convergence throughout the area in question. Two, a transfer mechanism to offset whatever economic divergences exist in the area. The eurozone currently meets neither of these conditions. Thus the eurozone's days look numbered. Couldn't be simpler. End of story.
What is actually far more fascinating is the question that the current predicament in the eurozone poses about Japan as a single-currency area.
For the present, Japan is indeed a single-currency area. Its national single-currency is the yen. The yen is used and accepted everywhere up and down the Japanese isles. But for how much longer can these conditions last? Japan is a single-currency area but economic convergence is by no means complete throughout the economy. Not by a long shot.