There will never be one single game, period! It's death by a thousand cuts each time a new game comes out a small percentage leave for the new. Trending analysis shows that destinys players shink more with each release (TTK sales show rentention of 23 percent of players from vanilla). With long content droughts people will leave and some will come back, but eventually it will boil down to a small base mixture of casuals and hardcores (I assume 350K to 500K) who will stick with destiny 1 till 2 is released. Depending on how long it actually takes to put destiny 2 out will be a considerable influence on its performance and unless it's a critical and user success upon release, the numbers will not be as big as the first destiny. Also if another game comes out, but perfects the formula and has great mechanics (destiny's only true standout feature) it could spell game over.