I have quite a lot of sympathy for this view. I can think of no genre that has been crushed and stunted by one writer's influence the way fantasy has been by Tolkien, and that isn't Tolkien's fault but that of the people who wanted more of certain aspects of his writing. More pages, more elves/dwarves/Medieval times, more scope (never mind depth), and so on. (Especially about 10-20 years ago. Things are changing, but slowly). Without one intepretation of Tolkien, there would be no Eddings or Brooks. Yes, there has been plenty of non-epic fantasy, but I wonder how well it does in terms of sales.
And I see the point about worldbuilding. It does seem that for some readers, there must ultimately be a D&D manual for any setting at all. Everything has to be known to the author and preferably explained to the reader, where in many cases there's no real point in doing that at all. The irony is that, in a lot of fantasy, it doesn't really matter who was King of Xland 200 years ago, or which forest is haunted, because it's just shuffling a very small pack of cards to rearrange familiar ideas.
In SF, the setting is often used largely as the vehicle for the novel's ideas, especially with writers like Philip K Dick. Fantasy seems to be much more reluctant to use these one-off settings. Also, it's often necessary to use shorthand in a novel. Characters arrive in a country full of spies, with banners everwhere depicting the party symbol and pictures of the Glorious Leader on every wall. They're obviously in a totalitarian state. Does it matter where the Glorious Leader came from? Not if the story is about something different but set in the same place. You might as well ask who built the castle in Where Eagles Dare. It might be interesting, but it's just not the point of the story.
Then again, epic sells, and backstory and worldbuilding of this sort feel epic.