One word or phrase to describe something good at start but then gradually becoming worse [closed]

Solution 1:

to deteriorate

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.

Solution 2:

running out of steam / losing steam

This describes something that had energy at the start, but is growing tired or becoming depleted.

burning out

Again, using the metaphor of fuel being used up.

getting stale

This conveys that the thing in question is losing freshness. Or in this case, the ideas aren't as good as before.

played out

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

Solution 3:

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.

Solution 4:

Phrases include...

This TV show is...

going to the dogs

going down the tubes

going to pot

going downhill

going down the toilet

hitting the skids

Lexico

to deteriorate, be in decline, degenerate, decay