we also have to remember that part of the reason things have changed and changed in many ways for the easier or the casual player is because of boredom.
Seriously, how long can someone play hardcore? Are the main raiders of Paragon and Enisidia/Nihilim/SWGamer (or whatever) the same as they were? just that one example of e/n/s should give you all the indication that we need. People are going to come and go from games. People are going to play until the game is no longer fun.
The advantage that Blizzard and Warcraft have created is that it isn't too hard to get in the game. I new player can come into the game and really in about a month be 80, and in about another month be fully raid ready and that's in ICC. And that also doesn't mean that the raid will suck because they have a 2month old rookie either. Time in game, while helpful, does not mean skill.
The problem lots of games have is that they die through attition(sp?) and there isn't a population base not only to replace those people, but those games haven't created an environment where new players can also prosper.
EQ lost people because of the Alt Advance levels. And in their forums people were constantly talking about ways to make the game easier, and while admirable, Sony stayed on course. People left through attrition and there weren't players coming up the ranks to take their place.
By the time Sony realized the problem, WoW came along and had basically stolen 75% of their population.
Blizzard, IMO, has seen this and is taking proactive steps to continue the game. And while it's been helpful for them to have such a great run and with little to no competition, that is also in part to them have adjusted the game over time to make sure they stay on top.
How would a lot of the population today compare Vanilla with Warhammer or Aion? Which would be the better game if things didn't change? If Blizzard didn't make the changes would Warhammer or Aion or something else be the game of choice?
And while many may say that WoW would still remain on top, I find it hard to agree. For example, how many druids would still be playing if they stayed the same as they were in Vanilla as basically innervate bots. Blizzard has changed this game hugely, from bascially making most classes and specs viable for playing, raiding, and to some extent pvp.