It's never been out of the question. In the years that they have been kicking the can down the road, all of the EU financial institutions have been preparing for this exact thing. You can expect Greece to exit the euro with little to no effect. The euro will of course rise, because the caliber of the remaining countries are much stronger.
The real issue is contagion. The EU doesn't want Italy or Spain to go down as well, because the markets/bond vigilantes love to pile on when there is blood in the water. Look at what happened during the Asian currency crisis, it was mass contagion throughout Asia that caused the subsequent problem.
That's also what, afaict, is keeping the problem from just being solved. Greece is small enough that it's quite possible to restructure their bonds into some workable package rather than kicking the can 6 months at a time. The IMF, for example, was floating a €50 billion debt haircut, which is not a ton of money in absolute terms. Alternate proposals would cap repayment in line with GDP, e.g. 1% of GDP per year, which would in effect amount to writing off a few tens of billion € per year (by letting it inflate away). But the EU is worried about setting a precedent that might impact a much bigger country with bigger debts, like Spain. (The other issue is just pure political constraints. Many EU countries currently have strong populist-right parties either in government or with an influential position in the government, like True Finns and Dansk Folkeparti. These parties are obviously against anything that looks like being "soft on Greece".)
IIRC, the effects of the late '90s Asian crisis didn't last this nearly this long though... The Greek tragedy/farce double feature has been going on for over five years now.
It's like Greece has been frozen into a creeping state of crisis. This can destroy an entire generation. A rapid shock might have been preferable all along.
The EU has already said that the referendum is about staying or leaving the EU.
Leaving the euro would be the best thing for the country. You can't rehabilitate your economy if you don't have control over your own currency (dropping the exchange rate, encouraging and influx of foreign money, etc). Look at Iceland, only a few years after their banking crisis, they are thriving.
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The real issue is contagion. The EU doesn't want Italy or Spain to go down as well, because the markets/bond vigilantes love to pile on when there is blood in the water. Look at what happened during the Asian currency crisis, it was mass contagion throughout Asia that caused the subsequent problem.