Nobody wants a grexit, not even Syriza. But something has to give soon, if Greece gets IMF on their side it might be difficult for the creditor nations to resist some kind of debt relief, but what form and what size is impossible to know right now.
Grexit will happen because there are three outcomes and grexit is the not the worst case scenario for either side. Possibilities:
1)Greek bank run with no bail-out leading to collapse of economy and government
2) Writedowns/write offs of Greek debt leading several other countries to re-negotiate austerity terms and massive losses to sovereign debt-holders.
3) Grexit.
It's going to happen. The only real question is "when?"
You are assuming that the EU or Greece will choose the less bad option. Ballsy!
A more likely reason why there can be a grexit is simply because everyone fucks up and are forced to accept a grexit that nobody wants. That would be politics european style!