I disagree. To me, the adventure is about establishing the PCs in various social contexts: plugging them in with the Veiled Alliance, (depowered) templars, nobles, and (emancipated) slaves. These are fertile grounds for further adventures, and I think one of the most important jobs of an intro adventure is to provide seeds for future adventures.
I also think it helps that the main thing the Big Damn Heroes do is mainly off-screen. Yes, there's a "cut-scene" where Rikus throws a spear at Kalak, but the actual fight happens deep within the ziggurat. The PCs have to deal with the situation outside. I don't see this as any different from "The castle is collapsing and you need to get out before it's too late" – it's just that the "castle" in this case is a life-draining ritual.
It also does kick off the setting meta-plot, but I think that in itself it's fairly inoffensive. Were 2024 Staffan in charge of the development of 1991 Dark Sun, I'd have frozen it after Freedom/Verdant Passage. Having one city-state having thrown off the shackles of their sorcerer-monarch and abolishing slavery makes for a really interesting situation. But much like Eberron, a setting is best served by having a myriad of potential things that could happen rather than having one specific thing happen and leading to another.
A worse example is also from Dark Sun: Black Flames. This has PCs being separated from their caravan by a sandstorm, and winding up in the ruins of Yaramuke where they get tricked by a 22nd level dragon (described as a "minor dragon in the scheme of things) who needs patsies to go get something from the ruins for him (he can't go in himself because some curse, so he needs 3rd-level PCs to do his work for him), and ends with a big fight between said dragon and a sorcerer-monarch who happens to be in the neighborhood because of Reasons.
I mean, you're welcome to that opinion, but I know that adventure really pissed off people, people with very low standards, people who tolerated other metaplots, back in the 1990s. So it kind of doesn't matter if you've got a view that's different (no matter how reasonable), because it somehow managed to rub people the wrong way. We were far from the only ones I know, too, based on discussions on the early internet, where pretty much no-one defended that adventure.
And I think it's pretty obvious why, re-reading as I just have (well glanced through). It's a really bad and annoying railroad full of assumptions even before that. Like, I think what the problem really was, was the adventure already pisses people off by railroading them
hard (a lot of DS adventures do, but it's one of the worst for it) and telling the DM to treat them this way and that way rather than naturally RPing or letting the PCs have much impact at all, all in order to absolutely force them to be on the Grassy Knoll in Dealey Plaza (as it were), which is very much not a natural place for them to be. You could have the same adventure without forcing this specific setup so hard - that's solely to because Troy Denning needed his awful, awful characters to be Big Damn Heroes in a way that was canon to the setting.
Re: difference, the difference is the castle collapsing is 99% of the time because the PCs either killed the Big Bad who inexplicably was stopping that from happening somehow, or pulled the "collapse castle" lever.
(I would particularly note that one of the issues with these characters is that most of them are
profoundly unlikeable, in an impressive variety of ways. I think word "dicks" would apply to most of them! This is a stark contrast to say, the Heroes of the Lance, who, aside from Tasslehoff Divisive-foot, are broadly "meh" to "fully likeable", which meant players who had read one or more of the books were further primed to be annoyed by this.)
Were 2024 Staffan in charge of the development of 1991 Dark Sun, I'd have frozen it after Freedom/Verdant Passage.
Personally I would have had the books be an alternative reality, just not reflected by the setting at all, but that wasn't the way things went in the 1990s. Have Kalak dead, sure, but give the DM like three different "truths" about how he died, as was not even uncommon at the time (though it did get more common as time went on). I suspect that's similar to what you're envisioning in practical terms.
A worse example is also from Dark Sun: Black Flames. This has PCs being separated from their caravan by a sandstorm, and winding up in the ruins of Yaramuke where they get tricked by a 22nd level dragon (described as a "minor dragon in the scheme of things) who needs patsies to go get something from the ruins for him (he can't go in himself because some curse, so he needs 3rd-level PCs to do his work for him), and ends with a big fight between said dragon and a sorcerer-monarch who happens to be in the neighborhood because of Reasons.
That does sound pretty annoying, I must admit, but exactly how annoying it gets would depend on how brutal the railroading was up to that ending.