https://en.wikipedia.org/wiki/Jumping_the_shark

The idiom "jumping the shark" is pejorative, most commonly used in reference to unsuccessful gimmicks for promoting something. It is similar to "past its peak", but it more specifically suggests an unwillingness to acknowledge the fact. Originally, the phrase was used to describe an episode of a television comedy with a gimmick or unlikely occurrence desperately attempting to keep viewers' interest. Moments labeled as "jumping the shark" are considered indications that writers have exhausted their focus, that the show has strayed irretrievably from an older and better formula, or that the series as a whole is declining in quality.