There’s nothing written in stone to say Greece can’t drop out of the euro, of course. The pivotal question today though is: Can the other euro members permit themselves to let Greece depart?
The answer to that I think is ‘No’. A Greek departure now would almost certainly forever poison the 'United States of Europe' project, which itself has always been the ulterior motive behind the creation and ongoing existence of the euro.
Why? Well, if you look at WWII as an attempt to unify Europe by force, then what triumphed against that attempt was the principle of peaceful change: In a word, democracy. And which country gave birth to the idea of democracy? Well, that was Greece of course, a couple of thousand years ago.
Looked at economically Greece then is entirely dispensable; but looked at politically Greece reaches down into the very DNA of the Greater European Project. To negligently 'permit' Greece to leave the euro now for merely monetary reasons would be to dishonour the driving democratic principle of the Project itself.
And this I think is the vein that the young leader of the leftist Greek anti-austerity faction, Tsirpas, is tapping into: It's the one vein that inextricably connects Athens to Europe's two most vital organs, Berlin and Paris.