I don't know about these "book" thingies you people like, but it's a clearly observable phenomenon in Sci-Fi movies.
Star Wars : OK, the first two sequels were cool. George Lucas had enough common sense to keep things fresh. The indicator that things were going to go awry : Ewoks. Lucas then waits 20 years. In the intervening period, he has had legions of fans consistently tell him he's a god. He makes a new movie, and he fills it with Ewok-principle marketable cutesy kiddie crap. An alternate analysis would include the Star Wars Holiday Special as the nadir, and incorporate the lengthy hiatus (and the need to make the 4th movie a prequel) as necessary to repair the Star Wars universe from this damage.
Star Trek : "Diminishing returns" sums it up. Trek hit its peak with later-period TNG. Then, with Gene Roddenbury out of the way and (inexplicable) cancellation of TNG, a small core of cast and production have pledged to kill Star Trek dead. Step one : "Let's make a show where they no longer boldly go anywhere!" Step Two : "Let's have one where they're boldly going like crazy, but are all personality-free, corny fuckwits!" Whoever dreamt up that fucking Neelix character should be forced to listen to The Transformed Man until they promise to do better.
Alien : the textbook example. The first two rock harder than Ozzy. Granted. Then the third, people griped about (although it was, in fact, rather good. Not as good as Aliens, but then not a Vietnam flick with xenomorphs either). But then disaster (i.e. the French) strikes! Alien Resurrection sucks so hard it's a wonder that the Alien queen bothers to eat its paper-thin "cast" of goofballs. Apparently this film isn't part of the official Alien continuity. It shouldn't be. The previously terrifying and camera-shy Aliens are paraded around like performing dolphins. And due to having the French (and Fox) involved, It makes no. fucking. sense. whatsoever.
Don't even get me started about Planet of The Apes.