I've been following the Greek debt crisis for at least five years, Greece's problem is that they absolutely refuse to stop spending money they don't have. Remember: Greece has never practiced real austerity (cutting deficits to match receipts) since they joined the Eurozone. Not once. (By contrast, Estonia did eliminate their deficit, and as a result started recovering from The Great Recession quicker than other EU economies.) Greece merely slowed the rate at which they were going more broke (or at least pretended to). Despite being right at the edge of complete national bankruptcy, Greece continues to insist that there will be “no wage or pension cuts” for government workers.
Greece lied about their economic situation to get into the Eurozone, lied about it before the crisis broke, lied after it broke, and continue to lie now.
Keep in mind that the past four years of bank loans from the ECB have not been to save Greece. What they were really designed to do was to keep the card game running long enough to let EU insiders and favored national banks unload Greek bonds, and to reduce their exposure to Greek default risks long enough to put European taxpayers onto the hook in the inevitable event of a Greek default. They pretended to save Greece, and Greece pretended to reform. And now here we are.
The adoption of the Euro hastened and deepened Greece's crisis, but was not the central cause, which was their refusal to stop spending money they didn't have to prop up their extravagant (even by European standards) welfare state. This modern welfare state has now become more sacred to voters than the capitalist economics that make it possible. As Mark Steyn put it, "People’s sense of entitlement endures long after the entitlement has ceased to make sense."
The problem is that with declining demographics, the cradle-to-grave European welfare state is unsustainable. Greece and the rest of the PIIGS are discovering that first, but birth rates are declining all across Europe, and modern welfare states are unsustainable without a new generation to stick with the bill. Most economists believe that Greece will never be able to pay back what they've already borrowed.
Syriza was elected on a platform of ignoring basic economic reality, but they've finally run out of people willing to loan them money to spend. The risk of a Grexit is already priced into all the European markets, But leaving the Eurozone doesn't provide relief for any of the Euro-denominated debt Greece already owes, and there's no guarantee European markets would even be willing to exchange refloated drachmas for real(er) money. And since it's hard to see any sane institution buying Greek debt after a default, Greece's government would undoubtedly start printing drachmas like mad and trigger hyperinflation.
If Greece was willing to pare back its welfare state to much saner levels, they might have a chance to slowly dig their way out of the crisis. Since they refuse to, they're in for a whole lot more economic pain...
