If they went around saying there engine is not finished and most concepts seen in 2013 were not going to be in the final game I would expect people not buying the game. They might have bought it, but would price it lower than a full game. If the original content was shown it would not have seemed as a big enough game as it is now. Destiny 2 is where they get to put that story into the game and make things in the 2013 trailer happen.