The closest thing we got to the idea of Radiant AI is probably Dwarf Fortress.
But entirely goal-driven (and thus unpredictable) game AI systems like this are usually at odds with story-driven gameplay where the outcome needs to be deterministic (or at least "winnable") and the player is the hero which the story is built around (while games like Dwarf Fortress don't have a pre-defined story, and also no player character to take care of, and the whole fortress being wiped out because of comically unpredictable events is a large part of the "fun").
Another similar game to Dwarf Fortres is Song of Syx [0]. It's more accessible then DF and I think they can have up to 20,000 entities active in the world at a time. The world map is pretty huge, and the player gets to control a one group among many. Every entity in Song of Syx is individually modelled, though probably not in quite the details that DF is known for.
That was also my thought. How does the world behave 100 hours into the simulation? If half the town residents have managed to get themselves killed by guards and some of the shopkeepers are gone, it's a bad outcome. Complex sims have emergent behaviors that are hard to tune.
The other thing is a bit more subtle. It's a big open world and all NPCs need to be active continuously for that sim to work. So you have a big N to squeeze into a tight per frame CPU budget. Also, things like path planning or object interaction only work if some information like object positions and pathfinding maps are kept in memory the whole time for the entire world. This sounds very challenging on a 2005 era PC.
The thing about Oblivion that the simulation tends to run up against is that hitpoints and death are an abstraction: a real human would die much more easily from the injuries that get inflicted, but also a real human would avoid a lot of problems in the first place (and have families that would take over their shop, and live in cities with more than twenty people hanging around). You run into trouble when one part of the simulation is taking things as symbolic while another part is taking it as literal. If you want it all to be literal you've got to be willing to go super deep into the emergent simulation.
In time since Oblivion we got games like Divinity: Original sin 1/2 where you can kill pretty much every character in the game and it will still be finishable.
The essential NPCs could also be flagged essential, or maybe have a variation of that flag where only way given character dies is if say 1/4 of the damage dealt to character is from player (so NPC can't accidentally kill important NPC basically).
Also, radiant AI can also just... not run on the plot significant NPCs.
Finally, Bethesda games aren't known from main story being the main selling point.
I think it's more than essential NPCs though. Already in Oblivion those couldn't die anyway (Morrowind was the last TES game where you could get locked out of finishing the main story if you killed the wrong NPC).
But fully emergent behavior would likely destroy some player's experience in other ways - towns without shopkeeps, most quests ruined, little staged moments going away, etc.
Some games in the Ultima series did, but Morrowind didn't, which is why Radiant AI was developed in the first place. The first chapter of the article is about that.
There's probably some mathematical way to express that... it'd be interesting to look at Todd's mythical "Radiant Economy", create a dynamical system model/game-theoretic mode, and try to prove that in the long run everyone doesn't end up broke or a millionaire.
I think Veloren has a sort of dynamic economy where NPCs trade in and consume goods. Well, maybe not NPCs, but at least settlements as a collective, or something like that. I'm unsure of the details, but I remember prices being different between settlements, and prices changing based on local NPC inventories.
> create a dynamical system model/game-theoretic mode, and try to prove that in the long run everyone doesn't end up broke or a millionaire.
Simply ask yourself which factors in the real world lead or don't lead (depending on your political stance) to this outcome, and you likely have found the relevant factors that you have to include.
To be fair, the velocity of money can be be significantly higher in a video game, and you're much less likely to have innovations reshuffling the market. It seems inevitable that extremal states would be more prevalent than real life.
With the point "the velocity of money can be be significantly higher in a video game" you actually outlined a serious problem (and a potential solution):
The rest of the in-game economy (including its pricing) doesn't fit the money circulation velocity, thus we get problems.
But entirely goal-driven (and thus unpredictable) game AI systems like this are usually at odds with story-driven gameplay where the outcome needs to be deterministic (or at least "winnable") and the player is the hero which the story is built around (while games like Dwarf Fortress don't have a pre-defined story, and also no player character to take care of, and the whole fortress being wiped out because of comically unpredictable events is a large part of the "fun").