So, you mean like Lost in Space turning into the show about the old man, the boy and his robot?
Or Happy Days being taken over by Fonzie?
Yeah, it happens, but it also probably saved those shows. As Michael pointed out, it's tough to keep it fresh and having a character break out and steal the hearts and minds of your viewers might be a real blessing.
It happens all the time, but it's not always a bad thing.
I don't think "Quark" ever really lived up to it's potential, nor did "Salvage One." I mean, in the pilot (of the latter) they are gathering space junk! It was all downhill, boring, earthbound-junk from there.
Heck, even Gilligan's Island! "Not a single luxury" Yet there wasn't anything the professor couldn't make out of coconuts and bamboo: batteries, dental floss, shark repellent, satellite launch system... Except a patch for the side of the boat!
They were hardly living without luxuries.
How about Fantasy Island? I think that may have moved closer to the premise near the end. Roarke was, at first, fulfilling fantasies as you'd expect, building up sets and flying in a secret guest from the person's past. By the end of the show, he was really using some kind of voodoo magic and creating their fantasies with a magical sound effect and a wink, with a perplexed Tattoo standing nearly apoplectic at his side, "But boss! How did you...? I mean what did...!? Aren't they dead...?"