Ketchup and Mustard are different sauces with different tastes and ingredients. They are used for different types of food but you can put them together on hot dogs. Empire builders can fit in with economic and political development and vice versa. They may be different games but they aren't incompatible.
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You are comparing mixing ketchup and mustard to mixing two fundamentally different games ... this is game development, not bloody physics. Your analogy does not apply to this scenario. It applies to a scenario where you take Victoria 2 & EU 4 game files and drop them all in the same folder ... it won't be a functional game, as matter of fact, it won't work at all (lots of files are named the same across both games and they would overwrite each other and make BOTH games unplayable).
I think this argument is pointless because you're just refusing to understand. Developing a game that would be a mix of the two would take massive development resources, more than needed to make any of the separate games, because the mechanics would need to be dynamic to adjust depending on the era of the game. And, once again, both game mechanics are massively different. If we simply combine the both games as they are now, it would create a game so badly optimized that you wouldn't want to play into V2's timeline because it'd be unbearable. The population calculations already take a toll after a hundred years of Victoria 2 play.
It would be really hard to use population mechanics of Victoria 2 for Europa Universalis IV, because the population information of that period is much more scarce and the developers would need to put lots of artificial numbers that have no historical basis. What's the point of that?