I personally think that the Halo series has kind of gotten less better every game. Only Halo one and two have the Halo rings in them and in Halo two they blow up. So what is the point of continuing the series without the actual moral of the story, therefore giving it it's title as well.
Well I don't know that you are referring too by the halo. I do agree that halo is getting worse every game but not because that the "halo" is not in the game. The point of the Halo is to get rid of the aliens completely. If the you think about other game series the killed off the main character but still continue to make games. One example of this is in Call Of Duty when Soap is killed. He was one of the main characters. But the Call Of Duty series continues to thrive. They do this to make the player try harder at a game or create emotion and attachment to the character.