an army of a million against 4 armies of several million isn't exactly good odds. And true, guardians can be reconstructed so long as their ghost lives. But what if the ghost is destroyed? There were dead guardians on the moon what were described as having none of their light left, so it is very possible that a highly weakened guardian would be unable to respawn. They have a theoretical immortality, but are not immune to falling in battle for good. Now, of course the fallen and hive will not have anywhere near the forces to take the last city. Perhaps not even if all of the enemies came together could they take it. However, you forget the biggest threat of all; the darkness. It is returning from the edge of the solar system, and, as shown in the past, has the ability to corner the traveler and crush it. The traveler and humanity barely made it alive after their first encounter with the darkness. The guardians exist to fight back this force, everything else is just in the way. If they can't keep the darkness at bay, then it truly will be the end.