Alot of people defend ROTJ and the Star Wars prequels out of sense of devotion to the brand name, just as a large number of detractors of those films attack them out of a sense of devotion to the brand as defined by the first two installments. Some people just attack the prequels as they see ROTJ as part of the brand but the prequels as not.
These are sectarian arguments, flag following for the sake of it.
It's like someone backing a church or a nation state just because of it's name and history rather than by what it actually does or how it functions in the here and now.
One of the potential hazards of continuing a story beyond it's initial state is any new additions colour or flavour the audience reaction to the original product.
Star Wars as a collected text is dragged down in the view of the general viewing audience by the failings (percieved or arguably observable) of one or more of it's component parts.
There is always an element of subjectivity here too (what works for one viewer may not work for someone else, one viewer may have different levels of expectation from a work than an another which would influence their reaction to each installment and to the greater canon).
Personally I find it more useful to take each installment seperatly and then later weigh it against what sit beside it.
The Terminator was a well made B-Movie inspired by (or shameless derivative of) a number of stories, mostly by Harlan Ellison (but he borrowed many of the themes from elsewhere too). It had a self contained and generally bleak (with a few rays of lightness) atmosphere and a brisk, tight narrative style.
Terminator 2 was a slicker looking comic book style B-Movie which took the bitter sweet, self contained narrative of the original away and replaced it with a can do, proactive survivalist mantra. It was compromised by the previous film's association with an actor who's rising fame and desire to play comic/heroic figures took the film into Roger Moore Bond territory. The director too wanted to be taken more seriously and the long monologues and heavy handed message laden narrative swerves only served to detract from the fun it still wanted to maintain from the B-Movie roots of the original film. Where the first film pulled everything in one direction this film tried to pull in all directions at once. The conclusion of the film (with the origins of Skynet destroyed but John Conner still alive) could only be reconciled by alternate timeline definition of time travel (countering the predestination model of the first film). So what were they really fighting for? If somewhere Skynet still exists to be fought and to send back the Terminator and Kyle Reese in the first film all the people who died in the second one, including the hero Terminator died in vain. The ending was clearly there to allow for further sequels as logically Sarah Conner would return to being any woman USA and her son would not come into being if they really could change the future (even the dropped alternate ending keeps old Sarah is aware of the events that have happened to her so somewhere those events have happened because of a seperate but related set of events she cannot prevent). So for me it didn't work either as a sequel or as a film in it's own right, it has some interesting moments and elements but they don't successfully come together well.
At least Terminator 3 (for all it many faults, which have been largely corrected in fan edits) returned the series back to it's fatalism and bitter sweet bleakness. It even acknowledged the flaw in the ending of the previous film, no computer company keeps all it project details in one location and all that Sarah Conner and Co managed to do was delay and slightly alter the timing of Judgment Day from their perspective. It's no classic but it's certainly not a chained down by star or director ambitions (Arnie actually does some real acting in the third film, he looks almost the same but is playing a very different machine). The theatrical cut was compromised by some strange humourous interludes but with those removed it's a much better sequel but not much of a film in it's own right.
The fourth one is compromised by a star (but not as much as Terminator 2 was) and a director who wants to make a name for himself rather than change the perception people already have of him as was the case with Cameron. Once again the studio allowed the story to be interfered with in an attempt to anticipate and avoid an audience reaction which it doing so actually caused (the story of the film and the making of the film have overlapped). The fear that without the linking star name and a bigger role for the new star name the series would fold created a distorted ending which spoils an otherwise solid film. It's like a shaggy dog story joke without the necessary punchline. But as a shaggy dog story it has more going for it and pulls in less directions (until the dreaded final act) than the second one does.
I'm actually beginning to wish they had put the Project Angel storyline in.
It would at least add something new to the series.
The Terminator in the second film observes that we are programmed for self destruction.
The idea that Skynet might try to re-engineer humanity to give it what it seems to want (freedom from death, pain and responsibilty and guided by all knowing, all seeing God like entity) and that the hero of the resistance against this New World Order is a machine with a human soul is bold new direction for the series to take. Sure people would be angered just by the idea itself (no matter how well it was executed) but people seem to be miffed by the film as it currently stands anyway. For a film which set out to be about the truth behind myth finding out that there was more behind Conner and Skynet than we were lead to believe would have made the film interestingly different but remain true to the source.
Sequels only really succeed when they go against expectations and forget about exterior pressures (which will always be there if the film is good or bad).
Godfather II and ESB are successful sequels because they give more than what than just a re-run of what went before but stay true to what went before.
As for The Wonderful Wizard Of Oz Baum's ending is actually very clever because it sets up Oz as a related but separate realm, the characters exist both in Oz as he scarecrow, Lion etc but they have their counterparts in this realm. Dorothy sees it as just a dream because it makes sense to her that way and because the adults back up her interpretation of what happened. As born out in the sequel where the realm exists without Dorothy having to be there.
Baum practically invented the idea of parallel universes in science fiction by doing that.
