So I support an MMO that wants to base its gameplay around heavy dependence on player cooperation/interaction. I don't think that any blanket statement should be made for all MMORPGs across the board. The issue with "End Game" is that it is usually done through a progression track. Once you are at the end game you run the end game track and are often not a part of the progression track to get there. This means over time you lose players doing the progression part of the game. Meaning that your necessary group content doesn't have the players to complete it because the trickle of new players don't have the wealth of end game players to pull from. So you need to make a game where the end game participates with the progression track or design a system that doesn't have that flaw in it to begin with.