What about Dracula? Frankenstein? Dr. Jekyll & Mr. Hyde? Those have shown some impressive longevity. I will admit, though, that those books are quite different from modern horror.
For works of horror to have any particular longevity they must either become part of popular culture or have some special philosophical or literary significance. The reason these works have survived is much more to do with Universal Pictures and Hammer Horror than their merit as books, though I enjoy all of them. Far, far, FAR more people can recognise the characters of Count Dracula, Frankenstein's Monster and Jekyll and Hyde, than have actually read any of the works in which they appear. Additionally, each work has particular things about it and sub-texts that make them historically interesting. Dracula is an interesting illumination of Victorian ideas about sex. Frankenstein was written by the daughter of one of the first feminists, whilst she was a teenager married to a famous romantic poet, not to mention being one of the first works of science-fiction, and being far enough ahead of its time to raise issues that are still debated in scientific ethics today. The Strange Case of Dr. Jekyll and Mr. Hyde plays on vivid and troubling psychological issues. Even then, these issues pale next to the popular currency given to them by their film (and before that stage) adaptations. There are other works of the period in the same genre with even more interesting concerns that are almost unknown nowadays. For example, Le Fanu's Carmilla, in my opinion, is better written than Dracula, and must be just about the first work in the modern western canon to deal with female homosexuality, yet today it is relatively obscure, living on mainly as the inspiration for the 'lesbian vampires' meme.
Stephen King, on the other hand, writes novels about evil trucks and demonic clowns killing small town Americans. Just about the only reason I can see for his novels being studied in the future is his curious obsession with the magic negro cliche. Just about the only 20th century horror writer I can see being studied seriously for a long time is HP Lovecraft, who seems to pretty much grow more in stature as time passes nowadays. Apart from him,
maybe Anne Rice.
Also, remember, popularity doesn't ensure longevity. Imitation, adaptation and influence and a somewhat arbitary canon (mainly decided by the tastes of educators and academics) are I think what secures the legacy of literary works. Grisham, for example, writes very popular books, but they don't have a particularly wide cultural influence, not on the scale of, say, Ian Fleming. J.K. Rowling, on the other hand, writes books that are not only almost inconcievably popular, but have had a significant cultural impact. She is one of the few modern writers who I think will live on in the popular imagination: I imagine this will actually be quite a small number, though of course many more authors will live on as subjects of academic study. The one really arbitary factor, which we cannot even begin to imagine, is the effect that future critical readings, philosophical trends and cultural shifts will have on the popularity of modern authors. Sometimes even being vastly popular in its day will not save a book. How many of you have read Pamela?