Modern MMO's are far more accessible which has a direct impact longevity. Players reach end game and level cap way too fast. The first batch of MMO's required massive amounts of time to reach either....and while time-sinks, hell levels, corpse runs, extremely limited fast travel and death penalties all suck, what they gave those games was longevity and difficulty. It took a long time to do a lot of things. But in fixing what was terrible about all those mechanics, the pendulum swung too far wide in the other direction. Players now burn through content at a rate much faster than companies can keep pace with.
Instancing has also been a blessing and curse. One one hand, spawn camping and racing SUCKS, but instancing lost a bit of that 'Massively' magic and cut groups of players off from the rest of the world.
For a long time Blizzard walked this balance with WoW and built a sub base that was unimaginable, but over time, the game began to take shortcuts and started shifting the direction of the game that had longer term effects on longevity (but continued having immediate big subscriber growth), impacting both the games economy, class balance, and itemization in ways that would be hard to repair but the effects of which would manifest slowly over time. Looking at WoW during Vanilla/BC and then again at Cata/Mists, you can clearly see the internal vision shift in many areas of the game. Blizzard redefined the MMO genre and re-wrote the rule book for what a successful and well built next-gen title should look like, but my opinion is that they began to lose their way a bit and what the game has evolved into has become challenging for the company to keep pace with even in the case of casual gamers. The content consumption churn is just too fast.
This shouldn't be taken as a sleight against WoW or the team that built and maintain it. WoW is a good game, and what Blizzard did was damn hard. That doesn't mean there are not lessons learned that we can reflect on and understand not only what made the game the success it is, but also understand some of the areas where things went a bit sideways and mis-steps were made.
Has the MMORPG genre peaked? Yes and No. Its evolved. There is still a lot of money being made from it, and it certainly isn't going away any time soon. That being said, the modern MMORPG player would not be a fan of the classic MMORPG. That type of game has died. The model that Everquest and UO were built on is dead, and in some ways, the model that WoW was built on is dying now.
