Season 1s are great, setup, some payoff, a bit of lead into the overarching story. Then season 2 to X. The heroes win and then lose in the final episode, cliffhanger to next season. People get bored. Final season is announced and they wrap up the show.
Once they realize they have a potential cash cow on their hands they do whatever they can to ensure that they can milk it as long as possible. S1 has no gaurentee of being a hit when its made so show producers put their all into crafting an enjoyable show. Once it becomes purely about extending its life as much as possible. Usually turning the story to crap in the process. I call this 'the Dexter effect"
Sounds like enshittyfication for series.
You see it pretty clearly with Stranger Things. In S1 each character has a specific purpose or role they fill in the story to back up the themes the show wants to explore, and they excel at that role. S1 is great, and they weren’t expecting to get an S2. But they did.
Now those same characters, with their specific roles? Well now they need to change (because you’re telling a different story), and they aren’t a super great fit for the new roles they have to play. It still kinda works, but the show’s themes become muddled and you’re banking a lot on the audience’s love of the characters now. Still a success.
Now we get to S3 and we have to change the characters’ roles even more! Entire storylines from S2 have now gone to waste, and many characters are far from their initial roles and don’t feel as interesting or compelling as they used to be - because they were never meant to get this far. They’re cogs jammed into new and unfamiliar spaces to try and get this machine to keep running. And it lurches and jerks its way forward, but it’s a far cry from the efficient, effective show it was in S1.