Pretty good article by the guy who wrote the now-infamous “shark” episode. But I think he’s a bit confused about what the phrase means, exactly. It doesn’t have anything to do with the referenced show’s popularity, or longevity. Rather, it represents the point that the show’s (or whatever) creative focus strayed so irredeemably far from what made it entertaining to begin with that there was no going back. It’s the show’s Rubicon, the point that history looks back on and says, “Yeah, that was pretty much the beginning of the end.” Like Happy Days, the show could still have strong ratings, and go on for years afterward. However, it was creatively spent, and had to resort to ever more outlandish stunts that just took it farther and farther away from the reasons people loved it to begin with.
That’s what “jumping the shark” means.