I believe his reputation is solid. What I wonder is whether, in the long run, he will be considered a classic of the genre, as Dracula and Frankenstein are considered classics.
I'm sure he will. I think he will be read centuries from now on. I'm not sure which books will last the longest or remain the most popular - It, The Stand or Misery seem more likely than, say, Needful Things or Tommyknockers.
The thing about him is that just the writing itself is so good. I'm sometimes thinking: ok, WHAT sets him apart from other writers, WHAT makes him so special. I have to admit it's not always the ideas/plots (although a lot of ideas and plots ARE very good); in the end it's the writing itself - there is something special about it which other writers don't (always) have: a way of making characters and situations come fully alive, as vivid as a movie (even more immersive sometimes). Even with the ones where I'm disappointed with the way a story plays out in the end, that feeling of immersion was always there.
I think it's that quality that people will still respond to centuries from now.
Plus, there is often a central idea to his books everyone immediately responds to: an outsider girl with special powers, a haunted isolated hotel, a rabid dog, a deadly car, a burial ground that brings people back to life, a scary clown-monster, a crazed fan.
Even if he builds these rather simple basic ideas into long stories, the basic synopsis directly connects to people. A lot of other books seem to have a more complicated (or maybe less direct or more veiled) synopsis to begin with, making people less inclined to pick them up.