Remember when Family Matters became "The Urkel Show"?
Now try Happy Days. The star of Happy Days was Ron Howard as "Richie Cummingham", But the dynamics of Happy Days was destroyed by the popularity of Henry Winkler's, "Fonzie".
The Rule of Thunb in entertainment says that the comic relief must never become the star of the show. When this rule is violated, the show's decline is very predictable.
"Fonzie" was a supporting character, never intended as the star of the show. By giving in to the character's popularity you derail the show by changing the relationships between the main characters.
"Richie" was the good guy, "Fonzie" was the bad boy. To make Fonzie the star of the show was to strip the character of his bad boy persona, and thereby destroy everything that led to his popularity in the first place.
Bowing to popular pressure is nothing new in television, but making unwise moves has caused the sudden demise of many TV shows.So, "Jumping the Shark" comes in many varieties.
Here's another Rule of Thumb violation: when "single co-star characters" get married in the show. The sexual tension that make such a show fun to watch is suddlenly all gone.
But the most famous violation: "having a baby / cute kid adoption". The list of TV shows that fell to their knees after the appearance of a baby or cute kid is surprising, counting some very formidable franchises.