People keep talking about reducing Greek debt to a sustainable level, but part of the problem is that there is not such level. Even at zero. The problem is that Greece is running a government deficit even before any debt service, so if creditors were to waive all of its debt, it would still need to be borrowing new money tomorrow. Debt forgiveness is not enough -- what the Greeks need is for Europe to write off all its debt, and then (having lost all their money on the old debt) start lending new money immediately. Note also that any bailout agreement reached this month will just put everyone back in the exact same place a few months from now.
This situation cannot be expected to change any time soon, for a variety of reasons from demographics (Greece has the oldest population in Europe, and a relatively rich pension system) to ideology (the current pseudo-Marxist government will never implement the reforms needed to turn the economy around, even if they promise to do so under duress).
With structural solutions unlikely, Greece has only the options of charity and inflation. Greece still seems to be hoping for charity, which they make harder by spewing derision at the same folks whom they are begging for alms. Europe, certainly Germany, is in no mood to be charitable any longer, but may still do so depending on their calculation about which action -- bailout or exit -- has the worse long-term consequences for keeping Portugal, Spain, and Italy both in the Euro and continuing to pay their debts.
Lacking charity, the only thing left is inflation. Some folks think I am advocating that option. I am not. The best possible hope for Greece is to slash its economic regulation, privatize business, and cut back on the public sector -- but that is not going to happen with the current government. Or maybe any government.
I say inflation is the only option because that is what balances the budget and "solves" debt problems when politicians are unable or unwilling to make any hard choices. It is sort of the default. If they can't balance the budget or figure out how to pay off debt, then inflation does it for them by reducing the value of pensions and outstanding debts**. This is what will happen with a Grexit -- a massive bout of devaluation and inflation what will greatly reduce the value of any IOU, whether it be a pension or a bank deposit.
Eventually, the one good thing that comes from inflation and devaluation is that the country becomes really cheap to outsiders. Tourists will flock in and olive oil will sell well internationally as the new drachma loses its value, creating value for people holding stronger currencies and potentially forming the basis for some sort of economic revival. My wife and I decided a few months back to postpone the Greek vacation we wanted this year -- too much turmoil is still possible -- and wait for it to be a bargain in 2016 or 2017.
**Postscript: This is exactly why the Euro is both immensely seductive and a dangerous trap for countries like Greece. Seductive, because it could pursue any sort of destructive banana republic fiscal policy it wished and still have a strong currency. A trap because it can no longer print money and inflate away its debt problems.