Players will always leave. New games come out. Some come back; others don't. New players come in, and those players and returning players are greater than the amount of players leaving. There comes a point where it starts leveling out until more players leave than are being replaced. DLCs come out to reverse that effect and the cycle starts over. When that's not enough, Destiny '2' will come out, and the cycle begins again on a greater scale. This will continue until there comes a point new content or new versions can't sustain the cycle - in other words, until the money stops rolling in. At this point, it doesn't appear that will happen any time soon.