Well yeah, if it's too time consuming or cumbersome for most people to replace three letters in a file, then most people are going to find modding province files very difficult indeed. 
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That's what I thought, too. I'm guessing there's a 'definition clash' between what we think of as 'Sandbox', and what the OP thinks it is. Which is fair enough, actually. It seems what he's looking for is a complete world editor, which would be a cool addition to a future V3 and the upcoming HOI4.
AFAIK a Sandbox mode is not the same thing as an editor, which is the thing that op is asking. Sandbox mode should mean a type of game where you can experiment builds, idea sets etc, all already doable by using cheats on the normal game. This imo makes it sandboxy 100%, while normally it still is kinda a sandbox game because you have no particular objective and except for some events you're not railroaded so you are de facto experimenting each game your ideas without necessary "losing" or "winning" the game when you decide you're done. A true sandbox for sure would need a god-mode (i.e. editor) to change the setup and experiment even more.
So, since the topic is the ability to edit the setup, then my answer is that it has been doable since eu2 (at least!) through mods. I don't know if devs think that the modding community is big enough to justify an editor, the only thing i can say is that the scenario editor of eu2 is still something i've to understand before i die while the basics of modding are very easy because it's basically changing plain text files and memorizing paths, there are also some guides on the modding section.
So, since the topic is the ability to edit the setup, then my answer is that it has been doable since eu2 (at least!) through mods. I don't know if devs think that the modding community is big enough to justify an editor, the only thing i can say is that the scenario editor of eu2 is still something i've to understand before i die while the basics of modding are very easy because it's basically changing plain text files and memorizing paths, there are also some guides on the modding section.