Either positive (https://tvtropes.org/pmwiki/pmwiki.php/GrowingTheBeard/LiveActionTV) or (https://tvtropes.org/pmwiki/pmwiki.php/Main/SophomoreSlump) negative (https://tvtropes.org/pmwiki/pmwiki.php/SeasonalRot/LiveActionTV).

The Mysteries of Laura starring Debra Messing was fabulously funny in Season One and then Season Two was terrible. It was like watching a completely different show. I was glad they canceled it after that train wreck of Season Two. Yikes.

Hangin With Mr. Cooper was hilarious its first season, when it aired on Tuesday nights. It was family-friendly (one episode featured a Full House crossover), but still contained mature humor.

Then it moved to the TGIF lineup, and they added Raven-Symone to the cast. At that point, it became cheesy and formulaic. Even as a child, I knew it wasn't the same.

Hangin With Mr. Cooper was hilarious its first season, when it aired on Tuesday nights. It was family-friendly (one episode featured a Full House crossover), but still contained mature humor.

Then it moved to the TGIF lineup, and they added Raven-Symone to the cast. At that point, it became cheesy and formulaic. Even as a child, I knew it wasn't the same.

I actually liked it more after it moved to TGIF.

Airwolf's final season produced by the USA network was a shockingly drastic lowering in quality from the first 3 seasons.

Ellen was another one that took a major turn for the worse in its 5th and final season.

Diff'rent Strokes took a nosedive when they added Maggie and her kid.

As did Silver Spoons when Jason Bateman left and was replaced with Alphonso Ribero.

The Streets of San Francisco lost something in that fifth and final go (1976-77) when Richard Hatch replaced Michael Douglas; Karl Malden's Stone character still kept it watchable, but it wasn't the same as when Michael Douglas' Keller character was with him.

Charlie’s Angels sees its more light in season 2 when Cheryl Ladd was added into the cast

The Facts of Life was less good after Season One. It was still okay until about the end of Season Six. I only like a handful of episodes after that.

I can only stand to watch the first four seasons of Family Matters; after that, it's like a one-note joke that went on too long.

The fourth season of Angel was terrible.

The later seasons of Charmed were ridiculous. At one point, they were chasing leprechauns through their house, and I was like, "WTF am I watching?"

The final original season of Roseanne was a trainwreck. I have never watched any sitcom less funny, or more creepy, than that last season. I read once that Roseanne's children were writing the episodes by then. I assume they were two or three years old at the time. Nothing else explains the drastic change in quality.

"MASH" really started to go downhill starting in season 8 when Radar left and Alan Alda started to write and direct more of the episodes. CBS should have cancelled it then.

"ELLEN" was funny the first 4 seasons; then in the 5th season she "came out of the closet" and it became the weekly "Ellen is gay" show.