No, I agree that writing in some of these things has fallen off a cliff. But again, it's just the status quo of whatever is popular to the consumer. If something sells for the minimum effort, why bother with the effort at all when you can pump something out quickly and cheaply for maximum profit? The gaming industry often shares parallels with films, and it's the same there, only with the actual thing that matters regarding what makes a decent game: game design. I've noticed it going down the shitter as well and largely for similar reasons. Ironically, games trying to be films is a thing I find that's a detriment.
As I said earlier, only regarding the gaming industry, you can sometimes pinpoint exactly where a series started to go wrong. An example I'll use here is Resident Evil, and specifically, Resident Evil 4. Now, by most accounts, 4 in this series represented the best of the lot. For some people it's their favorite game ever. I've not played it, but no doubt it's an amazing game. However, it marked the point where it was such a success and a high point with the fans, that the developers then decided that this is what the fans wanted and needed, and therefore continued on with making it less methodical, more action, less emphasis on engaging your brain, and it gradually went on to become more and more ridiculous until it reached the point where it was just a parody of what made it good to start with. In the case of the Alien series, you can see that it literally peaked by the second film and couldn't be taken any further. Cameron took the concept to the only place it could go and that was that. This is why Alien 3 gets so much flack, despite it actually being not that bad. It's just that it wasn't Aliens. There's just some things that run their course and burn themselves out very quickly, and no amount of talent in the writing department can salvage. Just look at Prometheus. It tried. It failed. Spectacularly. If we really hope to see good writing in things, then it has to come from a blank slate. Give us new stuff. Then again, that's a massive risk for them. Why? Because its new, and audiences generally go with what they know. It's a major factor why I absolutely fucking hated the latest Ghostbusters.
I know it's like comparing apples and oranges with films and games (more like apples and carrots, really) but it's a similar tale with a lot of these mega franchises. The difference is that its easier for them to get your cash with films because its less of a time commitment for the punter v a game that requires 40 hours to finish, and they can target times to release a thing when nothing else can compete in terms of brand recognition. It's an instant win.
I agree with hell of a lot of what you say and i think the games industry is a good parallel - Final Fantasy was ruined after X cos Square Enix followed the crowd and turned to more action based gameplay rather than a huge part of what had made FF so good, turn-based combat. It wasn't a case of bad writing really, they just followed the crowd in gameplay and it's suffered ever since.
i dont disagree that some things rightly come to their conclusion and should be a done deal - im not suggesting that every franchise or IP should get a next iteration in film/game, and some would be very hard to write for to add to the franchise. But as an example of is that the case or would good writing make it worthy to add another film, i'll use one you mentioned - Prometheus. Rightly panned, but why was it panned? Visuals were stellar, characters were pretty decent too (some supporting ones were poor admittedly, but that was the script - ie writing), it was the writing and in this case not the story, the story line was good if you look at the framework. However, the dialogue and lack of intelligence attributed to members of this 'elite' team and again cutting corners in exposition and lack of attention to detail killed it.
I remember the Prometheus thread on here well and certain things being hammered and (as i wasn't initially as disappointed as some) i wrote about all the assumptions i had made watching the movie to make it make sense, and someone replied that they 'learned' more and it all made much more sense from my post than watching the fucking film
if Scott had given Prometheus the care and attention he'd given Alien, im convince it could've been a top film worthy of the first two. So as someone pointed out, Aliens totally ramped up the action, which for them gave it nowhere to go - i dont believe there was nowhere to go if they decided to make another, it just needed the right story and a writer that could pull it off - Prometheus proves that was the case for me, it's all there, just appalling writing/cutting corners sunk it.
But back to your main point, blaming the audience, i will conceed their is an element of that - i recall TV before reality tv, that sums up your point in one. Forget good tv, the cheapest, most formulaic trash TV is being gobbled up, why spend money on TV slots when we can continually churn out this crap and they lap it up? Fair point. And Tv in general was ruined by reality TV that swamped our screens since.
But still, for me, for those who do get to write scripts, there's no excuse for a bad one, unless like i said before it was butchered for whatever reason, then that's down to the studio/director whatever. I dont know, maybe it's because im a writer myself and im of the belief that almost anything fictional can be good if the writing is quality, maybe that's why i lay so much at the door of the writers. Anyway, it's a pet peave of mine and i'll leave it there, taken up enough thread space.