You know how the monsters in the Percy Jackson series could be killed but could never die? It seems to be the same in Destiny, and I have an idea why. Vex are capable of time travel. That's a fact. And their tech is seeded into every world. So here's the theory: The Vex's time flickering breaks the continuity, causing certain key events to be accessible forever. However, the outcome can never change. So even if you fail to kill the Archon Priest, he can't appear in any later time because he's already dead. So why does every Guardian have to do every quest? They must implement themselves into the timeline. By being there in a successful version of an event, they become part of the story and can influence later events. What do you think? Am I just crazy? Or does my theory have some merit?