You specifically refereed to THE dark ages in your original comment, not A dark age. Large difference in the use of a and the. You will have to pardon me for not understanding you meant something other then what you wrote.
Also, by comparison to the world that was in Fallout, not even the real world today would be "modern". Going by that standard would be pointless, as absolutely nothing could compare, thus making any sort of comparison flawed.
So you are discounting political systems that have been used in established nations, both past and present, in the real world as not being nations in-game simply because? You also answered your own question..... all of the Legions fortresses, cities, and banners are east of the Colorado river..... which we don't get to visit in-game. Which is a problem, one even admitted by the devs, that doesn't negate that they exist in lore though.
Given that even Obsidian wants the NCR and Legion dead, I suspect it will happen eventually.
I doubt Bethesda would do it themselves though, they wouldn't want to touch the backlash of that with a 10 foot pole.