Don't know much about Jaws, but several of your other comparisons are imperfect in one important respect. While Batman, JP, etc. do indeed form a series, they do not have creative continuity. The Star Wars series does have one creator from beginning to end. So, are we to blame its lackluster final three (or four) episodes on franchise fatigue, or creative fatigue?
* Jurassic Park 3 was a made-to-order summer blockbuster. It was not representative of Crichton's vision or interests. JP2 was also heavily modified from the original novel. I think the film basically preserved the setting and two or three set pieces, then told a different story.
* Batman and Robin was really a sequel to Batman Forever, and almost completely divorced from the successful Tim Burton series. I view it as the second of the Schumacher series, and that series was always creatively bankrupt (or hamstrung, if you want to be generous to Joel).
* Superman III and IV were Salkind cash-ins IIRC, nothing to do with Donner's groundbreaking work in Superman I and parts of II.
The common denominator here is that these series crashed after the original creative talent left, or was dismissed. Not so with Star Wars: George Lucas was always there. It is with some perverse delight that I imagine the series might have been better had Lucas relinquished more control in the 1990s. Or maybe it would've been much worse. There's really no way to know.