Also, and this is really really really important to me, you're rather straying from the original statement. You originally spoke of events happening purely because the PCs show up. Now, you're expanding that out to "absolutely any detail whatsoever that wasn't perfectly planned out from the instant you conceived of the campaign," which...yes, I agree is a thing. You've made the statement valid by heavily weakening what it is you're saying.
It is just a difference of degree, not of kind.
Story events do not simply just happen because the PCs showed up somewhere. The world is not exclusively interesting in the location the PCs happen to be, and completely static everywhere else. That is what I was pushing back against. Specifically, you said:
World is not static outside the PCs of course. But often things happen specifically in location and time where the PCs happen to be, because the PCs are played by real human being whose whose fun is the objective of the game. This is like Adventure Design 101. If something happens somewhere where the PCs are not, and they will never even find about it, it effectively didn't happen.
That's a far cry from "people that the PCs interact with have their lives more fully fleshed-out than people they don't." What you spoke of is "the world only happens where the PCs are." What I'm talking about is the PCs digging into the world as they go. Big, big difference.
No situation "effectively just 'start(s)'" when the PCs arrive. Their arrival may catalyze a situation already in progress, or it may reveal more details than were already known, or it may draw out previously dormant actors, or (etc.), but it is absolutely NEVER the case that simply by showing up, Plot Happens to that location instead of some other location.
This is not hot how approximately 99.9% of GMs approach things. Back at Gertrude the goat. Farmer Bob's favourite goat Gertrude being kidnapped recently is a plot hook, and players will learn about this when they enter the village of Nirgendwo. (This will eventually lead them to learn about the were-chupacabras and their heinous plan to summon a greater daemon.) When exactly that when is doesn't really matter. Had they taken two weeks longer setting up a owlbear petting zoo at the Druid Grove before deciding to travel to Nirgendwo the were-chupacabras still would not have completed their ritual and Gertrude still would have just gone missing. Because none of these things are real. They're purely made up for the PCs to have a fun adventure.
And think of smaller things. When the PCs first time go to alchemist Balthazar's shop, they see thick colourful smoke coming out of windows, and meet the alchemist covered in soot and the shop in disarray, because they had an alchemical mishap. And this is just a fun way to introduce a quirky alchemist and inform the PCs that their concoctions might be accident prone. But again this thing just happens that exact moment because the PCs are going there then.
Now perhaps you're a highly anomalous GM and never do anything of this sort, though frankly, I really don't believe that. At leas the small scale stuff is so common and normal that it practically impossible to avoid. The games without illusionsm, at least in the super broad form you seem to be using, simply are not something that really exist, or at least commonly exist.
I mean, they may not be absolutely incontrovertible assumptions, but I think we agree that all but the most ardent, die-hard pro-illusionism DMs would agree that nearly all groups have at least one player that would feel rather disheartened by finding out that the game relied on illusionism regularly. I mean, literally every pro-illusionism discussion I've ever seen emphasizes the need to keep it a secret from the players. Nearly every person advising how to use it well makes extremely clear that, even if you don't think it would upset your players, you should avoid letting them find out, because it might and that would be very bad if it happened.
And that's really what completes my anti-illusionism argument. In the long-term, you can't maintain it, so you probably shouldn't. And in the short term, if everyone who advocates for it admits that you should take any means necessary to keep it a secret....maybe you should just not do it? It's one thing to preserve a mystery or create suspense--that's deceiving the characters. It's quite another to present false "choices" (not merely fictional, but outright false ones) and deceptive "consequences" (not merely made-up, but outright deceptive). I do tons of the former, there are several ongoing mysteries that the party is slowly working to uncover, and we semi-recently had an actual murder mystery situation.
No, just no. You don't reveal it in the same way like the magician doesn't reveal how exactly their trick is done or how it would disrupt the suspension of disbelief if a movie was paused to explain how the special effects were made. No one is deceived by this, everyone knows it is made up. This bizarre anti-illusionism stance is something I've only encountered online, and even then advance by handful of people who, frankly to me, seem to have no idea what the basic premise of RPGs is. The stuff is not real, the GM makes stuff up to entertain the players. This is not a heinous secret.
