What is the replay ability of most games? Average for top tier games is usually 6-8 months before you move on. MMOs challenge this with large expansions that are seemingly the size of the original game.
Destiny while it wants to be an MMO doesn't hit the expansion size premise. The outcome is a game that was made to last about a year and hopefully keep a small dedicated fanbase to stock the fires when the sequel is released.
The game isn't dying. It did what it was supposed and the some. They really should have just put out the game with all the dlc content and sign off until the release of destiny 2. Obviously patching when needed.