Your first MMOG is kind of like your first love. You never really forget it, even if you move on. Furthermore, that first experience often defines your expectations and preconceptions about how things are supposed to be. For me, the first MMOG I ever really got into was EverQuest. I could ramble on for hours, recounting the fun times and the good memories I have from that game. It felt so real, so interesting, and it was the coolest game I had ever played. Often, when I'm talking about something I think a game should or could be doing better, it's something EverQuest did in one way or another. I played it for four years on and off before I left it, and I still go back from time to time and play it for a few months. What did I leave it for? Take a guess.
Here's the kicker: This game that I loved so much? That I speak so fondly of? That I think did so much so well? It's the same EverQuest known for it's laughably bad quest system, insane grinds, ridiculously long camps, crappy interface, extreme difficulty to solo, broken mechanics, enormous raid sizes, XP loss on death, naked corpse runs, and a whole host of other complaints. And I still play it from time to time. If I'm that attached to my first MMOG, even with all of those negative aspects, how do you think people whose first game was World of Warcraft are going to feel about it in years to come? WoW took the EQ formula and fixed everything that sucked. There are lots of things that suck about WoW, but honestly, other than the familiarity, it's as much fun to pick up and play today as it was during my first day of beta. That's not the case for EverQuest. WoW did so much right that it's hard to significantly improve on the formula, as many studios are discovering--- see Scott Hartsman's comments on that topic in this interview, for example.
So what does all of this mean? Well, getting back to the original point I was making, I'm not convinced that any game will persuade first time MMOG players who enjoy WoW to leave it. Those people who are hunting for new games, who are tired of the old, who want something new... they tend to be people who already have hopped games once or more. When players find a game they like, a game that really grabs them, a game that defines the MMO genre in their minds and lays out what an MMOG should be for them, it's going to take either a serious blow to their game (Blizzard announces that they're done making expansions) or a serious improvement to the game style (on the EverQuest --> WoW level) to lure them away. I don't think that even Blizzard has really realized how powerful people's connection to their game of choice is (Michael has a great article on this from last week, actually). Had EverQuest 2 been as slick, polished, and fun as WoW at launch, you can be damn sure that I'd have been playing that for the last four years instead, just because the setting was familiar. I miss my Monk, Bard, and Enchanter classes terribly.
That's also why when I see a post speculating on whether over-saturation of the genre is going to hurt MMOGs, I can't help but feel like it's missing the real issue. I'm picking on Moorgard a bit here, but he's hardly the first person to consider that problem. The question isn't whether good games will be drowned in mediocre ones. Players will always find the good games. The question is whether the good games are good enough to pull players away from a good game that they already like, know, and are significantly invested in. In most cases, I'd bet that the answer is a definite "no." Why would players have any motivation to leave WoW, really? Unless they sit down to play your game (getting them to that point is a feat in itself) and the experience is so ridiculously much better that they wonder how they ever really put up with their old game, you're not going to keep them. Not for long. They're going to gravitate back to their comfort zone. You're much better off trying to attract a whole new set of players who can step into your game with fresh eyes, untainted by preconceived notions of what your game should be.
So when will the players leave WoW? Simple. Whenever another game comes along that advances the gameplay of the DikuMUD by another monumental leap, or when Blizzard closes the doors and shuts it down. If you can't bring that to the table, don't bother trying to lure their players away. At best, you'll get the lost souls already wandering from game to game, for whom WoW was just another pit stop on the road to the next interesting thing. At worst, your game will tank and ruin your company. Even if you can snag players for a while, they'll go home soon enough (wherever that is for them). The post-WoW MMOG market is exactly the same as the pre-WoW MMOG market, except you can't innovate on EverQuest and do what Blizzard did again. Don't be fooled. The best you can really hope for in that area is 200-500k sustained subscribers, and you have to be damn good to get that.
As far as I'm concerned, the denizens of Azeroth are there to stay--- at least for a while.