Mark Helprin's gone and done us all the service of advocating the idea that dare not speak its name: Rather than endlessly retroactively extending copyrights, why not make them last forever?
Unfortunately, he doesn't consider any of the various reasons that make this a terrible idea. Is it, for example, really such a bad thing that community theaters and schools all throughout the country (and, indeed, the world) can put on productions of Shakespeare's plays without paying stiff licensing fees? What if his heir and his team of consultants (I recommend Marsh) determined that the profit-maximizing license fee was really, really high -- something only the world's major theaters could afford, and something that they'd be willing to pay since his work is, to say the least, kind of well regarded.
Alternatively, one can imagine a world in which Herman Melville's great-great-grandson decides to release a "director's cut" version of Moby Dick and then embark on a campaign, à la George Lucas, to prevent the publication of the original version of the novel. He couldn't, of course, suppress the already existing print copies of the story which might continue to circulate, samizdat-style, for decades, but I still think there might be a problem. Melville fans and literary critics around the world would eagerly await Great-great-grandson Melville's demise and hope that his heir might be more reasonable.
You also already have an enormous problem of orphaned works, situations where nobody knows who owns the copyright to something, and where the person who owns the copyright may not even realize that the work exists. Obviously, the longer copyrights endure the worse this problem gets. Forever, meanwhile, is an extraordinarily long time -- we'd be drowning in orphaned works.
This last point is, in many ways, the crux of the matter. It would suck if my grandfather's novels -- or my grandmother's, or my dad's -- were to become orphaned in the future, or just unavailable because ownership of them passed into the hands of some jerk who didn't care about them. My grandparents are all people I know personally (or knew in the case of my late grandfather), but I couldn't so much as name all my great-grandparents.
Expecting N-th degree heirs to manage the oversight of cultural works properly is irresponsible. When things enter the public domain, by contrast, the practical impact is to put the fate of the work in the hands of whoever happens to know of and care about its existence. That, in turn, is a much healthier situation for world culture -- Shakespeare's works are whatever Shakespeare lovers make of them, which is how it should be.