The greatest irony here is that the more mainstream MMOs get, the more dumbed down they will get. Though, I believe we have already reached a saturation point in regards to "dumbing" down this genre.
Think about this: 10 years ago, who was playing MMOs? White males between the ages of 12 and 30. And not many of them either, mostly the loners and computer geeks. Now, everyone plays MMOs, whether it's Farmville or SWTOR or WoW or whatever.
The only way for game companies to reach everyone is to make everything easy and accessible. It's not like Call of Duty where you know your audience. I would wager there aren't as many housewives or grandpa's playing CoD as there are playing SWTOR. It has a more mass market appeal now which is great! But concurrently it also means to keep everyone happy, game companies have to create easy content that takes time rather than skill.
edit: My point is, the "stagnation" as you call it is 100% intentional by the game developers. You can't make a super innovative game that breaks the mold because noone will play it. A small subset of players will love it sure, but it won't have massmarket appeal. EQ never had mass market appeal nor did UO. Both of which are still around in some form and have a dedicated fanbase. They never achieved massmarket success like WoW. That's why wow is the "grandfather." It make MMOs more M than ever before.