I appreciate your thoughtful analysis, and agree in most spots, maybe disagree in some.
I think part of the reason for disappointment in sequels is one of the very things you cite: they often (intentionally) don't try to compete with or sometimes even continue the storyline of the original. They rely heavily on new characters. People who loved the original want to see more of it, even if it is similar, though I think we'd most all prefer it wasn't too similar. Of all the Oz-related movies, cartoons, etc. beyond the original, most were severely disappointing to me. Oz: The Great and Powerful was not. That doesn't mean it's on the level of the first, but overall I enjoyed it and it was respectful to the original.
Side note: Wicked in the theater was wonderful, I thought. It basically went right through the Wizard of Oz towards the end, all the while seemingly filling in some blanks pretty cleverly IMO. It set the same tone as WoOz. I'm not the biggest fan of the Broadway musical and went in a little skeptical, but really enjoyed it. This at some point inspired me to read the book, Wicked, which I absolutely did not enjoy at all. It was not the tone of the Wizard of Oz or the musical Wicked at all. It was dark, at times gross, what a miserable tale to tell! I strongly dislike it and would never read any of the other books in the series.
I never permitted myself to watch the remake of Charlie & The Chocolate Factory. The original means too much to me. I don't think it can be improved upon. I don't want any glimpse of a new one spoiling my watching of one of my all time huge favorite movies.
Conversely, I did go to the theater to see the new Poltergeist, and it fell extremely flat. Really poorly done. The original had so much more character development and made you really care about every person. It had a feeling of almost two movies in one - after they got Carol Anne back, it could have ended - main crisis resolved, right? But it kept going and was excellent to the end. The new one felt like they were just trying to hit every big point and get on to the next.
I'm sure there is a lot of emotional weight on all our parts that factor into these things regardless of the quality of a film.
The first 3 Star Wars films made (4,5,6) were equally iconic to me. I never understood the criticism that Jedi had gone downhill and the gasp that Ewoks were too cute or whatever. I loved it all. One of the biggest theatrical disappointments for me was episode one. But years later I went back and watched 1, 2, and 3 on DVD with a different set of expectations, and was able to enjoy them more in context. And I think part of the problem was the detachment from episode 4. Again, one of the reasons the first three films made were so successful was because we loved the main characters and the actors who portrayed them. None of that would have made sense in the prequels, but I think I was still looking for it somehow (even though we still had Yoda and the droids.) I wasn't interested in new worlds, Naboo, whatever. I wanted to dig into the pre-history of the worlds I already "knew." I realize the irrationality of this in the practical sense of film making. I still think that's one reason why these things often bomb.
The Harry Potter films all worked for me. But in fairness, those were very much planned to be what they were. Nobody tried to write the last set of three without the original author, or so many years removed from the original. It was a well done series (very much borrowing from the Star Wars storyline IMO - the young prodigy is at first unaware of himself, and is connected to the primary representation of evil - wands vs. lightsabers, etc.)
I very much enjoyed Saving Mr. Banks. I think that colors my willingness to accept a new Poppins. Plus I don't think I was all that enthralled with the original Poppins. I certainly didn't care for it as a kid. I watched it in connection with the Mr. Banks movie.
Which brings me to other factors: age and perspective. Big difference between someone who saw Star Wars in a theater as a kid, someone who saw it in the theater as an adult, and someone who never saw it in the theater, but on DVD or VHS. And as I am now in my mid-40's (yikes) 10 years means something very different to me than it did in my 20's or 30's. Ten years go by a lot faster now, it seems. Someone who is a kid today might get excited by another Spider-Man reboot. Me, I'm thinking: again? already?!?
With all that said, I tend to be a more open-minded and optimistic individual. It really bothers me when somebody poo-poos an idea before it ever gets a chance to be worked out. If I listened to everyone who ever told me, "that won't work," I probably would be managing a Walgreens right now or something instead of running my own business. If you want to make it work, you will find a way. If you want to make it out to be awful, you will.
One thing I know from the music business that can relate to this discussion is the people making the decisions are often not fans. They are guessing what their customers want, and that's why they so often get it wrong. When they reissue a certain artist's catalog with bonus content, and the bonus content falls flat - that couldn't really happen if a fan had been making the decisions and/or had full creative control (including licensing, etc.) Very often a release like that will come out, and the fans uniformly decry it, and the label is clueless - because they are not fans. This is just a fact of life. The biggest business people probably won't be the biggest fans or understand what fans want. Sometimes people are very well-intentioned, but just get it wrong. OK. Try again. (Haunted Mansion.)
And sometimes it's just budgetary restrictions holding things back.
Briefly, re: Tomorrowland: wanted to like it. Started out great, I thought. Fell apart towards the end.