Even if Greece grows at 10 percent per annum for the next ten years, it will not be able to pay its debts back. It's bankrupt. We better face the reality and not kick the can the can down the road. Greece should default.
Europe, and in particular NATO (the North Atlantic Treaty Organization military alliance), and the U.S. do not want Greece to leave (the euro zone) because if they do, other people are going to knock on Greece's door -- like the Russians or the Chinese maybe. It's very much a geopolitical game of chess that's being played.
I personally think it's not so much of an economic issue as a political issue.
I think that the ECB and European banks will have to take huge losses on their loans to Greece and bond purchases they have made (if it defaults). I think Greece is in a very strong negotiating position. If they don't want to pay what are you going to do, invade and hang them all up?