I for one think it's nonsensical to expect of Greece to be able to compete in the World Cup without several key players. Ask yourself how far Germany would get without say Schroder, Wagner and Obst.
Greece was missing 6 players. Out of whom I consider Giannis, Calathes, Sloukas and Mitoglou to be game changers. Even more so. Having lost the aforementioned four, even Kostas and Agravanis would have been key. And I leave out Dorsey because of Walkup.
You want to perform, bring your best. Otherwise you don't deserve it. Forget about Serbia. They are an exceptional case.
As for the future, as I mentioned before, even without Calathes and Sloukas Greece could have been much better with Giannis, Mitoglou, Kostas and Agravanis. But it would be great for Greece if in the next two years some guards will lift their level.
Right now ,Greece needs commitment and continuation. The quality is there. Any possible failure of Greece lies in there, rather than the performance of the team in a world Cup.