You pretty much reached the right conclusions, I think.

The literary horror genre and sub-genres are rather different beasts than the popular/trash horror movie genre. There used to be a kind of "second golden age" of horror movies, in the years of Alien, Rosemary's baby, the Exorcist, the Shining and so on, but after that the decline was sharp (with few exceptions, like Silence of the Lamb) Most of the output now is trash, of interest strictly to fans of horror movies.

If you look around, you will find the books that are the counterpart of the movies, so to speak (not that you'd want to read those).

Good horror literature has more to offer than the kind of easy, formulaic cheap thrills the movie experience does. King remains, essentially, an entertainer first and foremost, but through his body of work there remains a vision of American society (or his little corner of it). As an horror writer he has a peculiar to explore American life, but he does. Reviewers here occasionally draw comparisons to John Irving (not for the quality of the prose, of course!) and other New England writers. The horror elements aside (and alas, his average prose) aside, he's often not so very different from the themes, or angles, of mainstream writers from his generation and area. King is a very keen observer and commentator, and generally a good storyteller. The value of some of his best work was often stained by the genre stigma, but a lot of reviewers have seen past that. What people fear and jow they react to scary situations often tells a lot about who they are and what the believe, their personal or social flaws and strengths. That King is so succesful shows that first he is a very keen observer of his peers, very good at spotting the little details, at extrapolating from social realities to turn them into horror-generating situations and so on (so good in fact that as you pointed out, some of his horror stories extrapolated from his observation of violence in pop culture, or social exclusion/extreme frustration, or nihilism and so on have since seen real life counterparts. Another thing one of his novel, more SF than horror, has anticipated by many years is "reality TV". There is, and always as been, and element of speculative fiction in King's work, maybe not as deep or developped as some would prefer to see, but often dead on the money, for all that. It's often that or a mix of that and allegory that you find behind the surface of the better horror works. King remains one of the masters of the genre, even though the formal literary quality of his books is such that it's hard for many to put him among the greater writers of his generation (though a few do, and it's likely one day far ahead he gets put among writers like Stoker, Shelley, Poe and so on. It's hard to think of a more significant horror writer in the second half of the 20th century than Stephen King.)

The vast majority of horror movies are strictly meant for entertainment, and not very high quality at that because the main objective is to provide the thrills. It's almost like porn, the quality of the story or acting is to most fans most, most secondary to the success of the "thrills".

The Ring was crap. The original Japanese movie is remotely better, but while creepy it's not all that scary, in part because it plays on socio-cultural elements that are quite foreign to western culture (it played on "horror" folklore mixed with modern socio-cultural fears that you see not only in horror but explored in many other genre and medium in modern Japan, that has a peculiar relationship to modern technology. It's the most high-tech culture on earth, but they have their own reservations/fear about them. A lot of "Japanese monsters" appear totally ridiculous anyway to non-Japanese as much as western culture's monsters appear non-scary to Japanese. Humans are humans everywhere and some fears are universal, but cultures determine a lot how they manifest. In Japan's case a lot of monsters/fears are deeply linked to shintoism, and even when you know what it's all about, you still don't experience it the way Japanese do. Not that the few Japanese I know where that interested or scared by Ringu, though!). Like humour, horror is a genre that don't always export that well or easily.