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.
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.
Stranger Things doubly suffers because it’s horror. In the first season, neither the characters or the audience know what’s going on. The monsters are new and scary. The concepts are new and scary. The first season is incredible because it’s all unknown, and because there’s an almost cosmic horror quality to it.
However, by the end of the first season, both the characters and audience are experienced. The monster has been revealed and killed and, while it was tense and scary, the characters and audience know what to expect next time. The upside-down has been revealed and, while there’s a lot about the idea left to explore, there’s and understanding of what it is, how it works to some degree, how it’s linked to the real world, etc. Everyone has knowledge and experience. And with knowledge and experience, the horror dissipates.
So where do they go from there? Well all they can do is to make bigger, scarier concepts or to throw more of the same at the characters. More of the same can make for good action - see Aliens - but the horror element just doesn’t work any more, and it loses a sense of intimacy that a single monster brings. So the only way to try to maintain that feeling of horror is to go bigger and scarier.
Of course, the issue of intimacy remains. How do you have a huge, scary monster - far bigger and scarier than the first one - while still keeping it feeling both personal and intimate to our characters and having it feel “beatable”? And, well, you can see how Stranger Things struggled with that in season 2.
It’s interesting that you bring up Aliens because it’s a great example of how a character with previous history can fill a new role effectively, exploring different themes than in previous installments of a story.
But it also highlights the need for major changes - Ripley is the only carryover from Alien, and her character wasn’t really fully fleshed out in the first movie, so they had room for her to be whatever they needed her to be in a follow-up. She’s not a bad character at all, but she doesn’t need a whole character arc to fulfil her role. That’s not the case with most TV series because having thin protagonists at the end of a season generally doesn’t make for compelling or satisfying TV.
Aliens also works because they introduce new elements to the horror - the thought that there’s an intelligence directing the Xenomorphs is terrifying, and the threat it poses to life on earth is almost cosmic horror in its scope. You see that Stranger Things tried to take the same tack, but it was bogged down by its own lore and the limitations and having to work with a whole cast of characters who are experienced in fighting this very threat.
S4 was less of a mess than S3, and the worst parts of S4 were the cleanup from S3 — namely, the Hopper/Russia plot.
S4 could have leaned much more into the “Satanic Panic” theme. Dig into the “spiritual warfare” literature of the period — Frank Peretti’s This Present Darkness (1986) or the Mike Warnke “Satan Seller” scandal (exposed in 1985).