We can all think of many works that continue to be popular decades after their creation, but that's precisely because we only remember the popular ones. These are by far the exception to the rule, and we shouldn't tailor our copyright policies for them.

The intention of copyright is to promote the creation of works. Rewarding authors is the means, not the end. In the case of LOTR, Tolkien didn't write it to provide licensing revenue to his grandchildren; he wrote LOTR because the creation of fantasy languages and histories was his passion and he could make some money from it. A much shorter copyright would have sufficed to inspire the creation of LOTR. Furthermore, the creation of movie adaptations would be helped by LOTR coming out of copyright, not hindered. Disney's empire was built on adaptations of out-of-copyright stories. Shorter copyright would in many ways encourage the creation of more works.