I don't have any trouble keeping expectations in check with franchises, but it's inarguable that many went through some extremely rough patches, the ones artilector brought up included.
Almost all franchises produce a stinker or two, and, as in your case, someone has to be enough of a fan in the first place to see through the rough times. So if all Alien was to you was a space thriller, you're not as apt to be so forgiving about subsequent films not being as good or failing to compellingly build on the premise. I loved the sequel, but mostly for its entertaining characters, humor, and crazy-good visuals. The story? Meh. And I've felt that way ever since.
Prometheus was ambitious; unafraid to be its own thing while doing some unpredictable world-building. It's not a movie I'd watch again, but it was interesting.
As for other franchises, we'd probably agree where most went wrong. Rocky V, Lethal Weapon 3/4, everything after Die Hard 3, Iron Man 2, Schumacher Batman, everything after Superman 2, Spidey 3, Bond and Trek have been roller coasters of ups and downs, and the Terminator movies feel similar to Alien to me in that it's a weak premise that they never did a great job shoring up, leaving each film to live or die solely on its own merit. Still, they made a great one and a TV show that was surprisingly good.
Some of the revivals (the two most recent Rocky movies, new Trek and Star Wars) have really worked well. Some flew under the radar despite being pretty good (Renner's Bourne, John Rambo). The reboots have been hit (Batman) and miss (Amazing Spider-Man). And I'm with you on the modern Sherlocks. I like them, but it is kinda funny that there had to be 3 big, simultaneous Sherlocks, and does show a regurgitation-over-invention preference in film and TV today, even if they worked out in these cases.
I haven't seen Ritchie's Arthur yet, but I'm looking forward to it. I actually liked the one Fuqua did a decade or so ago. It was nice to see a different take with no magic; kind of a "what if this actually happened?" version. Underappreciated, in my opinion.
Anyway, my list of recent movies I liked might be a little askew. I haven't been able to go to the theater since 2014 (leukemia), so I've missed the cinematic, theatrical presentation a lot of tentpoles benefit from. I'll be able to get back to it this summer.
But I don't think you'd be shocked by my tastes. Hell or High Water was the movie I enjoyed the most last year, but as you said, a lot of that was having no expectations going in other than I like Bridges and that type of movie in general. Recent franchise flicks or franchise-starters? Dr. Strange was overrated, Batman/Superman was weak, Civil War was great, Jason Bourne was bad (which sucked because I loved those movies), the new Apes are great, Trek 3 was my favorite (way more original, less rehash), really enjoyed both new Star Wars despite generally not liking I/II/III, pretty bad Terminator, really enjoyed Fantastic Beasts...
I'm not sure the Hobbit/Rings movies count since they were both conceived and executed as one big story. I could have done without Hobbit 2 ending on a cliffhanger that was resolved in the first 10 minutes of the next one. That was bad. Each movie should stand on its own, and that one was incomplete.
Sorry for the novel, but it's a big subject.