Itself. Destiny will kill Destiny. Eventually, even the the most dedicated players will stop. Many already have. Negatively askewed RNG (raid/ToO), time-gated exotics, false promises ("Half a dozen new exotics for each class") and microtransactions all contributed towards this. And although Bungie will attempt to inject new life into the game through raid challenges and the Iron Banner, it may already be too late.