I guess difficulty may not be the best word when you dig into the details. The game does not have to be difficult in that it requires precise strategy, execution and it threatens a loss on the player. It is the immersion and challenge that vanilla HOI4 lacks as soon as the player gets a basic grasp of the game. Supply is one of the problems.
You are certainly on to something.
I will add something I consider to be of considerable importance in PDX game design in general. (I will wax philosophical for a few moments.)
Once you know the game mechanics of, say, HOI4, well enough, it's pretty smooth sailing in terms of winning. But time and again I see players struggling to learn the mechanics of a game like HOI4. (On another day, I'd post a series of links from people begging for help on the forum
with their invasions of France.) When Podcat revealed that the most commonly used difficulty setting was civilian, I realized that a substantial number of players, new ones in particular, don't learn the mechanics that quickly. And in some cases, they never really learn them at all.
This creates a weird situation for the Devs. On the one hand, I'd personally love to see more complicated
and punishing mechanics related to supply, attrition, and weather. Hell, I'd love a mechanic for ammunition production. But even if we assume perfect interface and perfect tutorials (that ain't gonna happen in any game ever), I question whether the mechanics I'd like to see would increase or decrease sales.
That's the real question, isn't it? Johan said (many years ago when EU3 was in development) that if he could make the company millions of dollars designing a game about pink ponies, he'd be interested. The Devs gotta earn, and I'm not their only customer. (Although I wonder what kind of game I could get PDX to design if I was Bill Gates and just gave them enough money to cover the entire development cycle of a AAA game to make a game just for me.)
And yes, you can make these mechanics rules you can toggle, but that creates an added burden on testing and quality control and AI, because you now have 50 more game states to test.