The historical evidence shows that Austerity causes demand contraction and often demand spiral, it has never in global history solved a budget crisis.
Austerity does cause demand contraction, but Greece is going to have to deal with it one way or another: there's no resources available for everyone to retire on a nice fat pension at age 55 anymore. It doesn't matter whether that comes out as a cut in euro-denominated pensions, or if that happens when people get drachma-denominated pensions: Greeks reliant on government funds must rely on less, because they're spending more than they take in, and it's pretty clear that lending money to Greece is less like lending and more like donating.
Now put a truckload of economic disruption on top of that mess.
The real way out for Greece was always securing a modicum of debt forgiveness and debt forbearance, plus enacting some economic reforms. The Greek economy is well-known as a sclerotic, over-regulated disaster area where you need to bribe corrupt bureaucrats to do anything and small-time rent-seeking plutocrats effectively own not just businesses but the right to operate important sectors of the economy. And the reforms were even going okay for a short while - real economic growth! - but the economic reforms proved unpopular with the leftists, and so this is what we get instead.
Glad it's not my country going through this.