It would be like saying 'pacman' is BETTER than the top selling games of today. Worse technology might be the REASON for it being worse, but it doesn't mean that its better.
The thing is, you can't really say that one game or movie is objectively better or worse than another. Beauty is in the eye of the beholder, as the saying goes, and any movie or game is ultimately only as good as the person experiencing thinks it is. It's a matter of subjective opinion rather than objective fact, and so I can't really meaningfully argue the point. If you like playing Pac-Man more than Skyrim, Pac-Man is a better game to you despite how anyone else feels about it or any other arguments that one could make.
I suppose there could be something in this. Can you give me an example of an old film you think does this well?
I think I did, with my descriptions of Psycho and NotLD, but I'll try one more. A perfect example of this in my mind is the original Hallowe'en versus the remake. The first one is unique and scary because of how minimalist it is. You don't know what Michael is or what he wants, only that he is going to kill you if he can. That unknown is absolutely terrifying to me. The remake decided to give him more backstory and just ended up removing the main source of "scary" in the film, changing him from a faceless personification of evil into a generic mentally-ill slasher. This is a big part of why I find modern horror less scary; these days, they spoon-feed you the scary. "Look how crazy this guy is! He was abused, so he doesn't know right from wrong! You should be afraid of him!". The original Hallowe'en had none of that; it left it up to you to figure out why this was happening, and that element of engaging my imagination makes it much scarier to me. But again, that's totally subjective.
I guess the question I have is why do you think these two are more unsettling than the (almost) copy cat films which have improved sound, visuals and IMO atmosphere.
Again, I'm not sure this is really a question that I can answer in a way that will change your mind. It's my opinion that The Exorcist is a scarier film than say, The Exorcism of Emily Rose, because I found it more well-written and thought provoking, with more compelling characters and a longer-lasting effect on me. IMO, what really makes a film scary is how long it stays with you and influences your outlook after you see it. Exorcist did a better job of that for me than any other demonic possession film I've seen so far. But again, that's just my opinion, and there's not really any way to convince you that I'm right (because I'm not, strictly speaking; I like it more, so it's better to me. You like other stuff more, so that's better to you).
I guess as a whole my misgivings about your post are simply that I think film makers have gained a LOT of new tools and I don't think anything has been lost. Sure there are a bunch of BAD new horror films, but I don't think that directers are failing on any of those unsettling/lingering effects which they still use IN CONJUNCTION wih improved visuals+sound.
As I've said, that's your subjective opinion and you are completely entitled to it. I don't agree, but that doesn't make either of us right or wrong. My opinion is that most modern horror films place all of their eggs in the visual basket, so to speak, and that they do this at the expense of of high-quality storytelling and psychological tension. Those are the two main things that I feel horror films have lost over the years. I honestly can't think of a film from the last 15 years that I actually found scary (and I watch a ton of horror), but even as a 30 year old man I will still be nervous in the shower for a week after watching Psycho. It just has a certain intangible quality that affects me in that way.
Sorry if this isn't the most satisfying answer, but IMO it's next to impossible to address matters of subjective opinion in a way that has a chance of changing your view.