...otherwise the player is left with no satisfaction.
A great man named Sir Arthur Conan Doyle wrote a great series called Sherlock Holmes. After many years of writing the adventures of his iconic character he wrote to his mother saying, "I think of slaying Holmes and winding up for good and all. He takes my mind from better things." His mother responded, "You may do what you deem fit, but the crowds will not take this lightheartedly."
This is absolutely applicable to Halo, Doyle justified the killing-off of Sherlock by saying that the protagonist was at the height of his story in the most important part of his life and once that was over he must be done away with because any story following the same character will be mundane in comparison.
Approaching us now is the Reclaimer Trilogy, we have been told by 343i that they have definite end plans for the series, we have also been told that John faces an "ancient enemy that threatens the entire universe", hyperbole or not this statement clearly shows us that this will be the most dangerous and important thing that John has to go through. This also means that in order for any satisfying conclusion to be reached, John, like Sherlock Homes, the never-enduring hero, has to die.
Let's face it, after this ancient enemy has been defeated and the universe is forever safe, what else is left for John to do? What else is left for a Spartan to do? I don't see any of them being integrated into normal society and settling down with a family after being honorably discharged, with no threat around they cease to have purpose and that brings down the character in the eyes of the player. For a super-soldier bred for a life of warfare to die anywhere other than the battlefield would be an insult, ergo in order for the Reclaimer Trilogy to deliver a truly satisfying ending to the character arc of John, he must die.