I get where you're coming from. However, I'd argue back that a specific NPC's and especially an "end boss" and iconic NPCs performance in the story isn't just about his on-screen performance, since the players presumably learned a lot about him through the adventure prior to that point, thus a more interesting backstory indeed adds to the quality of the adventure. Besides, I as a GM could do more work with a more interesting and relatable villain, it adds depth to his personality, to his goals and generally makes me more interested in him. I like when an adventure has interesting background besides the interesting here and now encounters. Makes the whole more lively to me. that, and i like to read stories, I like to read the adventure background in Paizo's APs, because even when the players won't explore the whole of it, it adds to my game.
My counterpoint is probably cliche, but: Star Wars. Without the expanded universe, we know next to nothing about Tarkin or the Emperor, and yet they're still
interesting to watch. They don't have complex goals, but I like watching them interact, and consider their performances
memorable. To me, that's more important than the actual "fleshing out" of character motivations. And that's what I mean. Heaps of background is all for naught if all you do is roll your eyes and groan when the character is on stage.
Having background is nice. Curse of Strahd has background. Background, however, isn't always
good. Background is space that could be used to have more events in the book. Background also needs to be relevant to the plot, or in some way accessible to players. If it
isn't, then it's utterly irrelevant wasted space.
Princes of the Apocalypse, as much as I like the premise, has that problem. The first chapter of the book is dedicated to the background of the adventure, but the characters have no means of accessing, or even
learning about the "Elemental Eye," making his presence utterly wasted space. And as far as I can tell, the only actual way to learn about the creation of the weapons is to backtrack out of the megadungeon (Which a number of DMs have had trouble motivating their parties to do), go on a side quest, and then ask the right questions, probably not knowing that the NPC can answer questions about the history of the elemental cults. Really, the adventure would do fine with a simple "Cultists are harnessing the elements for evil. Go stop them." If you need a bit more backstory, the prophets already have their own. It's very easy to strip out the Eye and Vizeran DeVir and not change the plot.
I don't know how Pathfinder is with making sure the background lore is actually useful and relevant to the plot.
Because it was advertised as a toe-dip in Ravenloft. Because Ravenloft isn't just the adventure, it's the setting. They would just need to write certain things that was written any way, but not in the way of discarding the setting, but working with it. That and adding a paragraph in the introduction, or in the end as a "how to continue the campaign" section. In a perfect way a short appendix, like in the end of SCAG about how to integrate the class options onto other settings. 1 or two page at best.
Would it have been really that much? That much space, that much effort? Wouldn't have been better to acknowledge the favorite setting of countless fans and one of D&D's most interesting worlds? To really bring it into the new edition, to open gates instead of closing them? Why it worked in the past but wouldn't have work in the present? Would it detract from the adventure?
I'm not talking about wasted space in a quantitative way. I'm talking in a
qualitative way. Going back to our analogy of the tunnel to Cormyr, it could also be a plot hook. The problem is: What is there to do in Cormyr? If the rest of the book never goes into doing things at Cormyr, then it's a plot hook that doesn't go anywhere, and should be exorcised from the book. Does that mean we're exorcising the Forgotten Realms out of OotA for not Cormyr exists? No, it's simply not relevant.
The same goes for the rest of the Demiplane of Dread. It's simply not
relevant. The story is about Strahd, and Barovia. The only time the other Realms are a factor is that Van Richten hails from Darkon. None of the other Lords are involved, none of them have any secrets to destroying Strahd, nothing. And it actually takes
away/ from the story, as you're now focusing less on the effects Strahd had on his corner of the world, the whole premise of the adventure.
And it's not the same as the "what comes next?" sections, as Curse of Strahd is relatively self contained. The "What comes next?" sections are about ways to continue on the story you're already going on, based on what's already happened. Curse of Strahd doesn't have that avenue to continue the story onwards with. Sure, I guess you could go on fighting the other Dread Lords, but why? In what way would the story be expanded upon to face another dread lord? If anything, it just takes away from the central element of the story, the glimmer of hope, by saying "Ha, joke's on you, there's an entire continent full of these guys, and no, you can't go home."
And no, I'm not really a big fan of the "Strahd comes back anyway and everything you've done is pointless" part of the ending in the current book.
(I recall something from an interview, or discussion, i definitely remember reading somewhere that Hickman always disliked the setting).
I remember hearing that, as well, but the closest I've ever found to confirming that was how Tracy was spiteful enough for somebody else making Lord Soth be involved in Ravenloft that he did what he could to contradict the two timelines.