So, lets say he resigns tomorrow, there's a new election next week and a similar government is delivered. What changes ?
Nothing really.
His position is pretty hopeless and he's highly unlikely to last very long but, essentially, he played his cards, he lost, and now he's trying to make the best of a bad job. However, he's still highly thought of by the vast majority of Greeks who put him in power so, even if he resigns some time this year, then I suspect we'll see him back at some time in the future.
Greek exit from the Euro is, for the moment, a smokescreen. Voluntarily committing the country back to the Drachma while the vast majority of the population are against such a move would be political suicide. Chances are that would lead very quickly to a new election and the very real possibility of a right wing government.
I think there's a good chance they will exit some time in the next few years, but I can't see it happening until it becomes apparent to the majority of the population that they'll be better off out than in. The only way that won't happen is if some decent deal is structured on extending or cancelling some of the debt, so that's a political question the Germans have to answer.
If he could have engineered a situation where exit was forced on Greece against his will, and lets face it, that was desperately close, then, politically, he'd have been in a much better place.
End of the day, he did what he thought was right for the country, or at least, what was least wrong. It's called pragmatism.