I think it's interesting that you mention westerns, SE, because I see those as a very similar situation in many ways. Just about every western movie or novel is based in the same period of history, sometimes called the cattle-drive period, which lasted for maybe 6 years. If you include the elegiac, "cowboy adapting to a civilized world" period, you add maybe four. That's ten years of American history that were so evocative and so compelling and so rich with possibility and atmosphere that they have defined a completely disproportionate chunk of our art and culture. And yet nobody ever complains about how all Westerns are the same, because that is all we really want our westerns to be.
Fantasy is suffering from the same problem--a single event abnormally shaping everything to come after--and yet fantasy is, by nature, about the discovery of the unknown so the limitations feel wrong.
I actually wonder if D&D might be more to blame than Tolkien, because it's responsible in large part for the institutionalization of Tolkien's creations: he made it, but D&D ingrained it in our minds.