DVD Beaver has a good point-for-point review, with screenshots, of Universal's attempt to re-dump all their Alfred Hitchcock movies onto the DVD market. In short: the new extras amount to about 15 minutes of clips, and they haven't done much work on improving the transfers, but there are some improvements. Most importantly, Vertigo has its original soundtrack restored; the previous DVD had a stereo soundtrack that was created by dubbing in new and painfully wrong-sounding sound effects.
Unfortunately, Universal isn't making any of these available separately, so those of us who would like to get just two or three are basically forced to get Torn Curtain and Topaz into the bargain. Which I suppose is the point, as far as Universal is concerned.
Regarding Hitchcock's post-Psycho output: Psycho was his last movie for Paramount (Universal now owns the rights, as with some of his other Paramount movies), and following its success, he spent the rest of his career at Universal, where he had near-Godlike status: one of the world's most famous directors, a personal friend and former client of MCA/Universal Superman Lew Wasserman, and an owner of a sizeable chunk of stock in Universal. He had the freedom to make what he wanted, and instead of answering to studio executives, he was more powerful than most of the suits.
And as so often happens with filmmakers who get a lot of freedom, autonomy and power (see Lucas, George), Hitchcock's career decline started from that point. The Birds was a success and has become something of a classic, but it had all kinds of warning signs of what was going to happen with Hitchcock's films: spotty casting, poor direction of the actors, big holes in the script ("Why do I go up to that attic?" Tippi Hedren asked Hitchcock. "Because I tell you to," he replied), and lots of tepid moments in between the big set-pieces.
These things all plague Hitchcock's post-Birds movies to one degree or another. Marnie could have been a great movie... with a better lead actress (or better direction of an inexperienced lead actress), and with a tighter script. Torn Curtain could have been enjoyable... if Paul Newman and Julie Andrews had gotten stronger direction, if the script made sense, and if Hitchcock hadn't listened to the people who told him that the film would be more viable if he fired Bernard Herrmann. Topaz could never have been a great or even interesting movie, but if Hitchcock hadn't let himself be talked out of doing Daddy's Gone a-Hunting instead... (You'll notice a pattern here: as with George Lucas, Hitchcock's autonomy didn't really save him from doing dumb pseudo-commercial things like dumping Herrmann or filming a Leon Uris potboiler; it's just that he was the one making the terrible decisions, instead of having them forced on him.) Frenzy, so clearly meant as a return to form, could have been just that if it hadn't been for the gaping plot holes, and the uneven cast, and the unimpressive music (as with Torn Curtain, a better composer -- in this case, Henry Mancini -- wrote a score for the picture that Hitchcock didn't use), and so on and so on.
The moral is certainly not that it's bad for a director to have autonomy or creative freedom. After all, it's not like Hitchcock at Paramount in the '50s didn't have the power to make the movies he wanted to make. But I am tempted to think that the kind of power Hitchcock had in the '60s, along with his status as an Institution, had a negative effect on his ability to correct some rather obvious flaws in the writing and structure of his films, or to direct actors effectively. There are some scary comparisons to be drawn between the kind of acting Hitchcock gets out of some of his actors in his post-Psycho films, and the acting George Lucas gets in his Star Wars prequels. In both cases it suggests a director who doesn't quite have the ability to communicate with humans.
Oddly enough, my favorite of Hitchcock's post-Psycho films is his last, Family Plot. Here he actually had a good writer (Ernest Lehman), a good cast (Barbara Harris, Bruce Dern), a good score (John Williams). It's not a great movie or anything like that, but it's the first Hitchcock film in a long time that's notable for something other than Hitchcock.
Addendum: On consideration, after writing the post below, I think I may have put too much weight on the "too much freedom" explanation for Hitchcock's decline after setting up his one-man operation at Universal. It might just as easily be true that Hitchcock's position at Universal gave him less freedom, not more. (It's not like a big stockholder in a corporation can then do whatever he wants with company money... not legally, anyway.) He certainly did seem, in his last decade, unusually willing to let the studio talk him out of projects, into others (Topaz), and into firing Herrmann. So perhaps the issue here is not so much an excess of freedom as a lack of artistic confidence, the kind that comes to a lot of aging filmmakers.
Howard Hawks comes to mind as another filmmaker who suffered a crisis of confidence, though it manifested itself in a different way. After the failure of Land of the Pharaohs in 1955 (his attempt to re-do Red River as an Egyptian epic), he took several years off, then came back with a hit, Rio Bravo -- a terrific movie where he and his writers basically revisited all the things they'd done in films past. (It's like a 140-minute catalog of favorite Hawks themes, lines and characters.) Having come back with that hit, Hawks spent the next decade mostly doing outright re-hashes of his earlier work, including two films that he turned into re-hashes of Rio Bravo when he didn't like the original script treatments. That's not a case of a filmmaker with too much freedom, but of a filmmaker afraid of failure, sticking to things that had already worked for him in the past instead of taking a chance (say, on Leigh Brackett's much darker first draft of El Dorado).
Also, there's just the simple point that few directors have made great films beyond a certain age. Indian summers are common in other arts -- music and literature -- but most great films are made by directors under the gold-watch age. (Exceptions, like Ran, are duly noted.) Of course that's partly because most studios won't hire a director over that age, but go over the filmography of many major directors and it does seem that directors, like athletes, tend to have a decline phase.