Archon said:
Agency theory of fun is all good and well, but even when the players make decisions you are reacting to them - not the world in some objective sense, but you, a person. Your web framework is a good idea, and probably works fine (my objection to it, noted in the comments, was that it took far too much time to build up) - but a web is still just a set of (sticky) paths, limited in number and no different in essence than a single railroad. In computer games we are given a number of options, dialogue choices, upgrade paths - they change stuff, but ultimately it all functions within a controlled world (the good, the bad, the neutral ending). This is, I suppose, my main opposition to everything you've been putting forward so far: you speak volubly of freedom of choice, agency, the mediating (not creative) role of the GM, yet when push comes to shove the game master is there pulling the strings, deciding when a fight happens, when a rout happens, who hates who and who adores who.
And this is exactly the way it should be, and must be. Because it is -only- in combat that chance rules (unless, once again, you use social rolls, such as in the White Wolf storytelling games - but these situations can be combative in their own right), in all other situations God steers the ship (the ship being every single other creature in the world).
I suppose that's, um, really all I wanted to say: player agency is ultimately illusory, and the GM's only task is to suspend the sense of disbelief while - yes indeed - spinning a story that people enjoy. So we're back to the beginning, where you think the judge role is the most important, and I think the storyteller role is the most important. But it's still fun to debate it!
If I may again speak on behalf of the author: I think you're making an error in your supposition that player agency is ultimately illusory. Consider the following:
Fiction is impermanent.
Let's assume I'm writing three books. In fact, let's assume I'm Tolkien, writing Lord of the Rings. Over the three books, I've got this character named Frodo, and this ring, and I basically want to write an epic journey about Frodo throwing this ring in a volcano. My goal is to write this story out, and at the end of the third book, Frodo throws the ring in a volcano and everything's good again.
Now, after the first book, my publisher approaches me:
"Bad news, Bryson. Frodo's not really meshing well with the mid-teen female demographic. We need you to give the rest of the quest to someone who the fans will better identify with."
"But...but I've already mapped out the whole story. How can I just change it so that the lead is someone else?"
"Don't be stupid. Just kill Frodo off, and bring in Frodina, Frodo's sister or something. I don't know, I'm a publisher, not a writer."
So I write out the remaining two books using Frodina, since Frodo died in battle trying to save his friends.
Now, consider this from our two perspectives. From YOUR perspective, as a reader, you have no idea that Frodo was ever /meant/ to throw the ring in the volcano. From your perspective, Frodo died, as he was supposed to die, and Frodina finished the quest.
From my perspective, the story changed, but you will never know the difference. Fiction is impermanent.
If you'd like a better example, and have seen Battlestar Galactica: Everyone had theories about who the DEADLY CYLON AGENTS were, but at the end of the day, it could literally have been anyone, and the writers could have tied the story together in any number of plausible ways, making it seem like /any random combination of characters/ were most obviously the DEADLY CYLON AGENTS from the very beginning.
In fiction, there's no impartial arbiter to make sure the events are internally consistent.
RPGs do not have to be impermanent.
You, as a world-builder, an RPG author, a DM, have the option of ACTUALLY making an internally consistent, permanent world. You present that world to the players, who interact with it, but part of that presentation needs to carry with it the feeling that you're impartial and will never sway things for, or against the players. More importantly, it needs to carry with it the feeling of internal consistency.
A better example than my previous examples!
Let's say you were DMing Battlestar Galactica. In the interests of actually creating an interesting world for your players to interact with, you determine ahead of time which of the characters are actually DEADLY CYLON AGENTS.
Two sessions in, your group, in a show of unexpected cleverness, believe with 100% certainty that one of the NPCs is a DEADLY CYLON AGENT. Subsequently, they decide to execute the NPC to be rid of them. And wouldn't you know it, they're right, the NPC they believe is a DEADLY CYLON AGENT is actually one of the ones you wrote as such.
You, as a DM, can do one of two fundamental things at this point. You can let it ride, act as the impartial arbiter, and let the players cleverness reward itself. Or you can take the dark path, and change the fiction so that the character they executed was actually a human all along.
Player agency.
This is where the notion of player agency comes in. The players ultimately had a choice to, say, execute this person. Though you contrived the circumstances in which the players encountered this person, their interactions, etc. etc., they ultimately had to make a choice.
The only way in which that choice would be illusory is if you fail to remain consistent in its results. If you keep the person as a DEADLY CYLON AGENT, then in reality, they've made a choice which has impacted the world. They've exercised their player agency, even if you have to come up with an internally consistent way to portray that, and their choice RESULTED IN AN EFFECT WHICH WAS RELATIVE AND CONSISTENT WITH THE CHOICE ITSELF.
If you change it so that the person was human, then their agency really was illusory, as there is no cause-effect relationship between their action and the results.
A quote from your post to end the argument.
"This is, I suppose, my main opposition to everything you've been putting forward so far: you speak volubly of freedom of choice, agency, the mediating (not creative) role of the GM, yet when push comes to shove the game master is there pulling the strings, deciding when a fight happens, when a rout happens, who hates who and who adores who."
No. The GM lays the FRAMEWORK for all of these things, but it's ultimately the players who determine how they interact with that framework. Maybe the players run away from the fight, or surrender. The rout happens as a result of the players' success in combat, or their fearsomeness. Those characters who hate and adore each other can have their feelings influenced by the players.
The only way in which the players don't have agency is if you take that agency from them; In a consistent, permanent world, there's only one way an NPC could react to any specific stimulus, and the DM needs pull no strings at all.