Stabs has a brilliant post up on raiding. He writes "Raiding has become a closed club. I know of no raiders in Rift who weren't serious raiders in WoW and other games."
While I don't know about raiding in Rift, I do confirm that even just looking at WoW raiding has become a closed club. And while Stabs is right in attributing this to the steep change in challenge between the pre-raid game and the raid game, I must also say that it has become a closed club because the people inside that club prefer the club to be closed!
Ultimately I do think that raiding as an endgame activity is doomed because of this, in any game. Any game or activity lives from positive word of mouth, from the players actively promoting their game or activity. The business rationale of making content which attracts the largest possible number of players to your game, and the closed club attitude of people believing that the less people do the same thing as they are, the more valuable that content becomes, are simply incompatible. Imagine a MMORPG where everybody who plays it discourages you from even trying, and then insults you as a n00b if you try anyway. If the current MMORPGs hadn't been designed by hardcore raiders, there would be no business reason to have raiding as an endgame in its current "exclusive" form.