I disagree. While specific books might come and go, the underlying trends do not. And a lot of these trends are... lets call it "mergable" with D&D style fantasy. That is, they can't be imported wholesale, but aspects of them can be merged with existing tropes.
Lets take out specific books and just talk genres:
1. New Weird
2. Steampunk
3. Romantic Fantasy
4. What can only roughly be termed "asian influenced."
5. Modern Dark
6. Maybe Modern Contemporary Goth?
7. Young adult
8. Urban Fantasy
So maybe China Mieville, Jay Lake, Mercedes Lackey, Miyazaki, Joe Abercrombie, Stephanie Meyer, J K Rowling, and Jim Butcher will turn out to be passing fads.* But even if that's so, they're just single examples from whole genres that I feel pretty safe in concluding will exist long after they're gone.
*I am very certain that some of the alleged classical greats are actually much less read than are Mercedes Lackey and Jim Butcher. I'd give Fafhrd and the Gray Mouser a 50 year head start and still expect them to lose a popularity contest versus Valdemar, presuming that the sample group was "people who read fantasy novels" instead of "males over the age of 30 who read fantasy novels."