Creating a new engine would have taken resources that were used to create actual content. Not sure if you've looked at the development timeline for Destiny 1, but they spent almost five years developing the Tiger engine, if I remember correctly. Not only would creating a new engine use up resources that are better put to work on things like abilities, map creation, AI, and story content, it's also not even feasible given the fact that they had a deadline to meet. It would be great if Destiny could have a fresh start to create a better engine, knowing all they do now, but it's just not possible, at least not with Activision on their tail. And besides, I don't think I would appreciate having to wait years just to be able to play on an engine more optimized for frames per second. And I also have doubts that even if they're engine was more optimized that it would allow the game to handle 60 frames on something like PS4. We'll only be able to see how much the engine really has to do with it when PC D2 is released so people can see their fps compared to similar games.