Happy Days and it had had a very long and successful run. Happy Days had nothing to do with water skiing or sharks. Fonzy was a cool motorcycle-riding tough guy, not a stunt man. The episode was untrue to what the show was about or what its characters were about.
It told all of America that the folks writing the scripts for Happy Days were out of ideas. At this point a mercy killing of the show was appropriate. And that's what happened shortly thereafter.
A story or series of stories has to be true to itself. Long running series, like Perry Rhodan managed by constantly escalating the stakes. But despite how large the galactic empire, Perry Rhodan and the characters around him were consistent with the space opera that starts with an astronaut finding a marooned extraterrestrial space ship.
Doctor Who has gone through nearly a dozen actors portraying a thousand-year-old alien who travels about in a blue police call box that's bigger on the inside than on the outside. Nevertheless, it has maintained a more or less internal consistency.
Different writers have been at the helm and they've pushed the story in their own directions. None appear to have tried to put him on water skis so far. I've thought the writing to be paper thin in places, but that's consistent with the cotton-candy nature of the show.
Then there's Castle. This show has been maddening because its writing is so inconsistent. I like the premise of a cop show with lots of eye-candy and Nathan Fillian living every scribbler's fantasy of making tons of money as a writer. It works well with a light-hearted comedy romance between the two main characters. It works less well when it forgets this and turns into a heavy drama. No, I don't believe these two are credible Jason Bourne material. Nor do I believe they're going to save the world from some international conspiracy. These episodes are painful to watch because the tension is so fake and the outcomes so predictable.
The audience has to believe in the characters. As the characters act untrue to their essential natures, the audience starts to doubt. When the doubt grows to a critical point, they're just a shark and a water-ski-ramp away from irrelevance.