They introduced an activity called a "raid" which rewarded gear that was required to hit max level/maximise damage output etc... From that point a line was drawn in the sand and this was no longer a conventional FPS. Whilst Destiny hasn't fully jumped on the MMO side of it, they've been dipping their toes over. The problem is finding the balance between the two, which is relatively fresh territory for a Console game. There is no "ideal" model for them to follow and are most likely attempting to figure it out as they go. Check any other major "evolving" game; their forums contain all the same arguments etc...