Oh, sure. And I agree with the point that not ever scenario has to be epic. Fair enough.
However, I think you're missing a bit of a point of those blog posts. It's not, "Don't let the BBEG get away", it's, "If the players legitimately whack the BBEG, don't rob them of that success". And, by mandating that the BBEG gets away, you are forcing specific stories on the game, and that's generally a bad idea.
Also, if you look at the structure of Keep on the Borderlands, it's essentially set up in very similar ways - you enter the cave, fight your way through to the boss monster, and kill the boss monster of that cave. Wash, rinse, repeat for each cave. It's not quite as open ended as you might think.
Yeah, many of them are more specific. And while I find the topic extremely interesting, it's not really my point here, other than to say that design goals have changed over the years and that specific "guidelines" such as those (which are also in the DMG) prevent some common tropes from occurring, or at least make it very difficult.
B2 gives specific directions to repopulate any rooms after 1d4 weeks, with returning monsters, or new monsters. It's designed as a living dungeon. In addition, there's another dungeon on the map that is specified for the DM to design, and also that the DM is to design what goes on outside of the mapped area of the wilderness. There are multiple aspects to the dungeon that are specifically directing the DM to expand the adventure beyond just following what's written in the module. Yes, there's a boss monster in each cave, and the design of boss monsters at the time was essentially one class of monster larger. So if you had goblins, the goblin chief was as powerful as an orc.
But we're really getting into a different discussion here. The specific play styles or differences aren't my point. I'm just giving examples, they examples aren't the specific point.
This all aside though, my biggest issue with "how it was in the day" is always a very self serving point. No one ever argues, "Well, we played this way back in the day, but, I think we were the only ones". No, it's "Well, everyone I knew played this way" with the presumption that that means something to the wider audience.
But, there are so many problems with that. If we're talking the "roots" of D&D, where do we start? With the stuff in the 70's where we have only a tiny fraction of the total number of gamers that boomed during the 80's? What's the point in that? We're only talking about a very small slice of the total gaming population - a population that apparently to some extent rejected the stand alone adventure in favor of the larger campaign. Why are things like Dragonlance (1983), Cult of the Reptile God (1982), Isle of the Ape (1985), Immortal Rules (1986), Ravenloft (1983), heck, even the Avatar Trilogy (as bad as they were) come out in 1989.
How is that not one of the roots of D&D? The game is what, 45 years old now. The Avatar Trilogy is almost 30 years old.
There isn't ONE root of D&D. There are a bunch of roots all making the tree. Arguing which root is more "rooty" is pointless.
By that argument,
The Phantom Menace is one of the roots of Star Wars. Roots are roots. Understanding the roots (which go beyond OD&D) can be enlightening and is an interesting discussion. But that's not my point. I specifically stated that I'm only relating my experiences, perception, and that it's necessarily a small sample size. So NO, I don't think it's how everybody played the game, and I also indicated that.
My message isn't "how it is in the day" so much as a simple observation that the focus of the game, by both design and presentation, has changed.
The OP was that adventures don't always have to be "save the world" adventures. Since 5e has come out, what type of adventures have been published by WotC? A two-part adventure to save the world from Tiamat. An adventure to save the world from the Elemental Cults. An adventure to save the world from the Demon princes. And one to save the world from the giants. The only ones that break this mold is
Curse of Strahd, and now
Tales from the Yawning Portal.
To somebody new to the game starting with the "official" published adventures, you have a pretty high likelihood of playing a "save the world" style game. That's not to say that they haven't incorporated other play styles.
Princes of the Apocalypse and to a larger degree,
Storm King's Thunder provide more of a sandbox approach.
Out of the Abyss looks back to the
Descent to the Depths of the Earth (and later adventures with similar approaches) with it's underground exploration. But the overriding story, the goal of the adventures remains "save the world."
This isn't an "it was better then" observation. It's just an observation of how the rules have altered the way the game is presented. Those who play indie games can tell you that the design of the game can have a huge impact on the types of adventures played by that game. They are frequently designed around a single intended play style, and the rules themselves help reinforce that play style.
D&D has never been designed for one specific play style. Well, perhaps Gary felt it was. But the reality is that it has always been more than one particular style. But over the years, certain play styles have been promoted more than others simply by the rules and the decisions of what to publish (and what not to publish). Whether you like 4e or not, it promoted tactical grid-based combat. What it did not promote was theater of the mind play. It wasn't designed for that.
I've also said that the current approach makes a lot of sense, especially from a business standpoint - a focused and consistent approach makes the game more approachable, easier to pick up and learn, and (from my perspective) appears to target the mass market, not the hardcore gamer. As much as I'd like more material that focuses on the types of things I like, they don't need to. I'll continue to pretty much buy it all, but modify it and pick it apart for my purposes. Not only would I be a very difficult audience to design "perfect" products for, but it would undoubtedly be a very small market. I would hope that people are playing them because they like them. That's kind of the point.
So, my perception is that in 5e, WotC is promoting the "save the world" style of gaming. Simple premise tied directly to the OP based on what has been published for 5e, especially when compared to the history of D&D and how other editions have been "promoted" through what has been published.
That may not be their intent. And perhaps it's not as obvious as 4e not promoting theater of the mind. But I think it is. I think they are playing up the epic style of play, which to some degree is at the expense of other styles.