Doesn't every new game show this kind of population spike/decline in the first few months? You have a bunch of people coming to the game at first, and then the number slowly goes down as people get bored with what they were playing and move on to something else. And this happens for pretty much every. Single. Game. Some have different rates obviously, no man's sky for example took a nose dive that has yet to be bested to my knowledge. And then you have some games that start and stay strong comparatively because, well, the game developers knew what they were doing and did it quite right (Overwatch) but even then you still have that same pattern of a large population at the start, and then it dwindles until it has stabilized with its core playerbase. Destiny 2 is still in the decline phase for now, and I think that if it stabilizes at a number higher than the first Destiny game, then it will have succeeded and done alright.