His movies still make tons of money and tend to be quite popular, regardless of actual quality. He will never again recapture what The Sixth Sense was, but I don't think he or any of his fans are really concerned with that.
I wonder if the mistake that some make is to look at his work as if he is writing something "great" (e.g. to be remembered in the history of story-telling / film-making), but on the other hand there's something about how they're marketed that screams that they should be taken as such.
For me, watching 'The Sixth Sense' for the first time in the cinema was an enjoyable experience that climaxed in the twist, but when I look back on it I felt that the twist was all there was to it.
I used to enjoy 'Unbreakable' more, there's a little bit of class in how the villain is portrayed in a way that very plausibly mixes the surreality of the comic book genre (e.g. comic-book villains wearing garish outfits) with reality. Since then though I've found myself watching the DVD less and less often to the point that I ended up giving it away. I wonder if I should watch a speed-run of it with only Samuel Jackson's scenes
Part of the problem is that I just don't think Bruce Willis ever had the acting chops to really carry a film (I'd be surprised if anyone could realistically name a good film that if it wasn't for Willis it would have bombed).
I couldn't bring myself to watch 'Split' because for the trailer it seemed like a very garish way to depict mental illness. Same obviously goes for 'Glass', and I've become less interested in his work as time went on. I have to admit that the trailer for 'Trap' intrigued me a bit.
I don't think M. Night Shyamalan is quite as low-brow as say Dan Brown, maybe a bit better than Brett Ratner? I have 'Red Dragon' on DVD but there are some scenes in it that just have no class in whatsoever (like that stupid final ending scene mentioning Agent Starling).