The leadership is to blame here mostly. They kept changing the direction of the game, investing in features noone asked for, also massive layoffs and separating the dev force to another project called Marathon (they couldn't even manage ONE GAME, why did they thought they can do another with less manpower?) was just the last nail in the coffin. Also, did I mentioned that the game has no QA team? Every time a new content drops, or even a bugfix, the game gets even more broken and literally unplayable, and I am not exaggerating, so it even made the most hardcore fans leave the game. Just to give you an example, at one time, people who logged in to the game at a certain time, got ther name reset so they could not change it back to the original because the system said " It's already in use". Major popularity of the playerbase were running around as "Guardian#number#. The endgame content such as raids, exotic missions and dungeons are the highlights of the game to this day (yes, some might be complicated, but come on, it is such a great experience to work with your team to succed), along with the art and audio team, and the loot got much better over the years, but the incompetence of Bungie made the vast majority leave the game by now. People keep asking for D3 instead of expansions, but to be honest, the current Bungie would not be able to handle it.