They are doing it for its good. Following the same story, sometimes a game gets repetitive. First 3 games if you remember well were awesome but if you think about it the story didnt change much from each other. (go destroy the Halo ring , fight covenant in first levels , then fight the Flood ).
The ideas soon run out, and the fight with the covenant cant go forever. So they moved the story toward a new objective. The fight with the forerunners. I personally love where this is going. Because it shows how massive the halo universe is and that its a fresh storyline. So im looking forward to Guardians.
Other games that did the same thing and succeeded: Bioshock, Saint Row ( the jump from 2 to 3) , Red Faction, Wolfenstein , Tomb Rider , Infamous etc.
Games with same storyline that failed: Call Of Duty, since the 2nd modern warfire it has been the same shit , not just graphically and game mechanics but storyline too , they just changed the era where its set and the main characters and then its all , go there blow that , kill those , infiltrate that , sabotage that , and the ending is with the main antagonist nearly killing you with a knife just to kill them in a ridiculous way.
