It is entirely reasonable to believe that a country that never lost a single war for 300 years who provided a better quality of living and economic growth than anywhere else in the world, gave local authority and minimal taxes while providing safety and security unprecedented throughout the history of the world could have united at least a majority of the world, and it is completely possible that the combined forces of most of the world could militarily conquer the rest of the world.
You're ignoring how historically every empire that's grown even remotely large enough to be considered a possible world conqueror has fallen, which seems like a very systematic trend that is a testament to there being an optimal equilibrium for the size of a state. Sure, we can always say that world conquest is physically possible, but based on what we understand about the dynamics of states and societies it remains seemingly implausible.
I wish people would stop confounding narrative and mechanics all the time.
Sorry buddy, but I'm going to have to heavily disagree with you here. For a game so centered on emergent systems and player-designated victory conditions, the only thing that separates mechanics and narrative is the player's mind. While that distinction works alright for many more traditional games, it breaks down for EU. Since there's no non-interactive narrative cutscenes, it's all just one big mechanical system, and the player decides which mechanics they wish to incorporate into their internal narrative; this can vary between players. Some players, like myself, might choose to utilize historical plausibility as what decides whether a mechanic qualifies as making a narrative contribution, and I'm sure other people have their own criteria. This is a much more robust way of seeing a game like EU than the artificial distinction between mechanics and narrative as separate parts of the game.
Historical, to the vast majority of forum posters here, is just a word they use to justify their preferences for game mechanics.
I think it's more of a question of levels of abstraction and simulation, at least when I personally use "historical" (or more ideally "plausible") to justify certain mechanics over others. Take base tax vs. simulating population growth/decline in a province: the latter is indisputably more historical because base tax is an abstraction that did not exist historically, and I happen to feel that higher historical fidelity improves the experience (because my play style centers on creating a plausible narrative rather than optimizing mechanics). I feel that history was a perfect system in which everything was balanced in one way or another, and we already have a wealthy understanding of general historical processes, so I don't see the need to create novel, gamey abstractions that comprise potentially-less-plausible systems. Yes, the game has a limited scope and we'll have to leave stuff out, but we can do that and still retain the system's core elements instead of abstracting everything beyond recognition.
"Historical" means exactly nothing in terms of game mechanics. History was crazy and chaotic and not rule-based in any sense we could implement as an abstract game system, which is why there's always a case to be made for the historical plausibility of any game mechanic, and the same case can be used to justify any side of any issue.
So you would conclude that the study of history as a rule-based system is meaningless? Well, you're entitled to your own opinion, but there are many prominent historians who would disagree with you. To me, all the great men and the exceptional incidents of seeming arbitrariness ("things almost happened X other way if not for Y") get a disproportionate amount of attention because they are just that: exceptional and phenomenal. The vast majority of history on the other hand does follow a causal pattern akin to a rule-based system.
At the very least, even if historicism has no impact on the performance of game mechanics in a strictly gamist sense, what about the narrative you always distinguish? Surely most people here can agree that historically plausible mechanics are preferable in this regard, and is this not equally valid grounds for judging mechanics? It seems you operate in a hyper-gamist manner, which is fine, but you've got to understand that there are other ways of approaching the game. A game, especially one like EU4, is so much more than just a game, and really the main reason we call it a game is for me because terms like "interactive play system" are a lot more unwieldy. There are multiple ways of interpreting the same element of a system; is it a game mechanic, a narrative point, a simulation rule, etc.? You seem to think that you have the authority to distinguish the boundaries between such aspects of the "game", when in reality this can and will differ from player to player; just because you consider something purely a mechanic and thus it should only be evaluated in a gamist light doesn't mean that other people must see that thing in the same manner. It's subjective.
You can't objectively say that any proposal is or is not "implausible" by drawing on historical cases, because it depends on the subjectively chosen scope and detail of your analysis, and it depends on what you subjectively choose to use for a stopping rule.
There is a very big difference (very big, not to be understated) between arguments about "historical" cases, and arguments about historical processes. I personally agree fully that the former are completely biased and worthless at the end of the day (and if they're all you mean to dismiss then I'm sorry for misinterpreting you). But I feel that we do have enough of a system-based understanding of history to decide which mechanics better approximate particular historical processes better than other mechanics do, in a generalized sense. Citing historical cases is like trying to reverse engineer an equation from a series of points, and to me that's not productive. It's better off to approach things from a top-down perspective, and while there will always be exception, at least that way you will capture the majority of cases.
So please everyone, stop bringing up historical cases, and focus more on the historical processes. The way I see it, cases are the grounds for in-game events, and processes are the domain of mechanics. Do not conflate the two.
And no one is right or wrong. Trying to decide whether Game Mechanic A or Game Mechanic B is more "historical" is like trying to decide whether Idea A or Idea B is "heavier." Attempting to let "historical plausibility" determine game mechanics (or the other way around, for that matter) is at best an unproductive framework for discussion, and at worst pure nonsense, since weight is not an intrinsic property of ideas, and it depends entirely how you subjectively choose to translate "weight" (historical plausibility) to ideas (mechanics).
I think I've mostly addressed this in the above paragraphs, but what would you advocate instead? What is your productive framework of discussion that you prefer? Something to deal with maximizing "strategy", I'm sure... but is not an impossibly complex and balanced system a la history the pinnacle of strategy? It seems to me that the historical agents who actually dealt with those systems were far keener strategists than we'll ever be with our incredible abstractions. So then what is it? Accessibility, ease of use? I say to leave that to other strategy games. I don't know what else you're advocating.
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All of this said, I don't believe the OP's suggestion is particularly "historical" or anything, as I have stated in a previous post in this thread.