There's no reason why the Euro zone should explode because of one smallish country leaving the club.
Then, it's very different from Cyprus, there aren't many wealthy people with big assets in Greek banks.
Most if not all Euro zone countries have financial problems.
Greece and Portugal already caused trouble to Euro zone, Greece leaving might start a trust crisis or something
Let them see what happen when Greece default and Greece must ask to the market to loan money to the government.