There's something I don't understand here. Some of the biggest creditors like Germany seem to be actively rooting for Greece to leave. Surely they understand that means that no one, themselves included, are going to be paid back. So at this point, does anyone really believe that they give a rat's butt about the amount they have already sunk into Greece. If they do care, they have a strange way of showing it.
From their standpoint there are two likely paths. First, Greece stays in the euro. In that case Greece will require another hundred billion or so before either everyone stops pretending this money will ever be paid back. The other option is the Grexit. Greece will still be a charity case, but perhaps less of one as the crashing drachma cheapens and thus encourages domestic production. Also, chances are very good that the IMF and/or USA pick up more of the additional lending. The creditors will also send a strong signal to the other PIGS nations that one cannot default and still enjoy the benefits of the EuroZone and the euro.
After one accepts that the money already loaned to Greece is gone, never to be repaid at any significant level, then the prospect of Greece exiting the EZ becomes more attractive. The biggest wild card I see is the chance that if not propped up, Greece becomes either a communist hellhole in Europe's side or intentionally becomes a Russian vassal state. If Greece is forced out of the euro, then they may not wish to continue as part of NATO either, and given the partial annexation of Ukraine, NATO can't be wild about the prospect of a Russian naval and missile base in Greece. That is why Tsipras has been so visibly courting Putin. It remains to be seen whether the EU can be blackmailed with this and if so, for how long and how much. And of course, whether Greece can ride the bear without becoming bear chow.
None of us have crystal balls (save perhaps those who owned 60s Triumphs with the wire tank racks) so we not only don't know what will happen, we can't know what should happen. ("Should" meaning best for Greece and its creditors, not some form of cosmic justice to punish the Greeks or those who loaned them money.)
EDIT: A bit more good news and bad news. First the good: talks continued for 14 hours, stretching into Monday morning. Thus we know that both sides are serious and do not consider it hopeless. The bad: Greece is apparently asking for another hundred billion euros over three years, with no payments; the European nations are refusing a third bailout unless the IMF is the major player, the IMF is refusing a second (for them) bailout unless the Euros forgive a major portion of the debt, and the Euros are refusing another write-off; even assuming Greece gets enough to keep its banks afloat, the nation also has staggering debts outside of the bailouts, such as over a billion euros owed to pharmaceutical companies; and without the bailout Greece will have to issue IOUs this week to meet pensions and wages. Note that this is all from leaks so YMMV.