For example, a TV show is very good at first two seasons, but afterward the quality starts to decline and in the end the overall rating becomes disappointing.
Then we can say this TV show is "word requested here".
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e.g. This, once well-received, TV show has deteriorated since the introduction of a new set of presenters.
An idiom is "to go downhill".
e.g. This, once well-received, TV show has steadily gone downhill since the introduction of a new set of presenters.
This describes something that had energy at the start, but is growing tired or becoming depleted.
Again, using the metaphor of fuel being used up.
This conveys that the thing in question is losing freshness. Or in this case, the ideas aren't as good as before.
If the ideas explored in the show have been reused too many times, it could be described as "played out".
Single-word options: waning, declining, dwindling, slipping, tired
Whereas it has a more distinctive meaning, you might want to consider:
Jumping the shark
In particular, this term is popular in TV culture, since it originated from a sitcom (Happy Days, 1974–1984). It is similar to 'past its peak' in the sense that both phrases imply that the TV show in question was once successful, unlike for example 'deteriorate'. However, 'jumping the shark' is more dissing towards the creators, since it is typically used to highlight failure in maintaining successfulness, quality or popularity of the TV show as the result of having exhausted focus and good ideas for the creation of new content. Whereas the term is mostly used in response to a specific event (like in its original usage), its purpose is to point out deterioration of the show as a whole, gradual or not. See also Nuking the fridge, a term that has been coined as a reference to 'Jumping the Shark' in response to a disappointingly unrealistic scene in the 'Indiana Jones and the Kingdom of the Crystal Skull' film.
This TV show is...
going to the dogs
going down the tubes
going to pot
going down the toilet
hitting the skids
to deteriorate, be in decline, degenerate, decay
All the other answers are good but played out is also a contender.
It is described by the Cambridge online dictionary as
tired and no longer having power or effectiveness
In its literal sense it applies to sports people who are no longer as good as they once were but metaphorically it is applied to many things including TV and radio shows; theatre shows; political policies and manifestos and even bands. When applied to shows in that way it means that the have lost their freshness and relevance, sometimes because times have changed but sometimes, particularly with situation comedy, because all the comic potential of the concept has been exploited.
A series that has become stale and lost its appeal can be said to be
past its sell-by date
- Literally, to be past the date by which an edible item should be sold before it spoils.
- By extension, to be past the peak of one's abilities or career.
It is used generally to mean that something is not as good as it used to be, and should be got rid of. In the case of the show: cancelled or closed down.
Fizzle or fizzle out
to fail or end feebly especially after a promising start
Used in a sentence: The first two seasons of Prison Break were brilliant, then it fizzled out.
From the online macmillian dictionary, there are multiple words to describe something becoming worse:
to become worse, or to make something worse
to become less or worse
to become worse
if a bad situation deepens, it becomes worse
to become much worse or more serious, or to make something do this
to become worse
to continuously become worse, more, or less
to gradually be in a worse situation than before
go down PHRASAL VERB
to become worse
come to the boil PHRASE
if a situation or feeling comes to the boil, it starts to become more serious or dangerous