Ah, what a delightful topic. I find this concept very interesting, as well I should, since it is my intention to publish in fantasy.
His article makes a surprising number of good points. I have noticed that in fantasy, there is as much an internal division between 'literary' works and 'non-literary works' as there is in mainstream fiction. It's actually an interesting subset, since fantasy (along with SF) is currently engaged in a lengthy battle for relevancy when regarded by the mainstream fiction world. Many in mainstream fiction tend to dismiss anything genre-related. SF and Fantasy, however, argue that they can be literary, and try and remove themselves as much as possible from their cousins romance, western, and mystery.
SF has been far more successful in its argument than fantasy. Fantasy's main problem is the one that Mieville explains in the beginning of his essay--the fact that it is caught in so many redundancies. Re-exploration of the heroic ideal has become the overused weed of fantasy writers, used because its easy, and because it sells so well (as Sprig pointed out.)
The literary fantasy writers get annoyed by this, as they have right to be. I postulate that it might go back to how difficult it is to write a successful fantasy book without falling into the stereotypes. I've tried it myself, and found that the book I wrote that used the heroic ideals was far better received than the one I considered original. In addition, the bulk of the material on the shelves keeps the minority of the clever fantasy from being noticed by the mainstream world.
However, both of these problems are something of an exaggeration. I have met many sf and fantasy editors. They seem to genuinely like original, interesting work. They come back to one simple theme--if you write a good book, it will sell. I think fantasy is in a transition period. In the decades following tolkien, Fantasy and SF were regulated to the pulps, where their escapist natures were a necessary part of their ability to sell. This 'Lifeline' for writers was a necessary part of their stories. They needed to include these ideas, and play off of Tolkien and Lewis because the world just wasn't ready to support serious professional fantasy yet. (Tolkien and Lewis were both professors, and didn't have to live off the sales of their books.)
A generation passed, and the Fantasy 'renaissance' came into existence. This began with people like Silverburg, McCaffery, and (yes) Eddings, who were willing to explore a little bit more. Look at Eddings, for instance. No elves, no dwarves. Yes, there was the same heroic plot, but many of the cliches were getting abandoned.
Yet, at this same time, it was very hard to abandon this old feeling of what fantasy was. The 'New Pulps' of TSR and its clones began churning out fantasy reminiscent of the old days, and as the younger generation--the first ones to really experience fantasy as a pervasive professional genre--read, they became indoctrinated.
Only now are we beginning to get tired of these ideas. The newest fantasy is becoming more and more exploratory, and is being more and more well-received. Just as SF escaped its buck-Rogers/Asmiov Robots trend, so Fantasy will overcome its roots--it just may take a few more decades.
Now, as for the core of the escapist argument, I don't believe it holds very well. Arguing that Tolkien and Lewis weren't interested in other issues implies that things such as exploring culture, history, and folklore are not worthy literary goals. In the very least, Tolkien's work is an excellent look at mythology and how people use and create it. There is also the issue of the ring and Frodo's growing dependence upon it. Yes, there is a neat tie-up of an ending, but reading is as much about the experience as it is about the conclusion.
