Playing some more HoI3 lately, I'm coming around to the idea that this was actually a sensible design choice. Spending 20 hours or so nursing a plan along into fruition isn't all that rewarding in SP, since the AI can't put up a coherent defense.
Well, yes. The AI has been very beatable in all versions, and has done some really whacky things over the years, or chooses to do nothing at all. That being said, I suppose the AI can't be too good or nobody would want to play against it.
My point about the expenditure of time into the game was really speaking to two different types of players. That is, those of us that appreciate the finer points of attack, defense, planning, etc., and those of us that want instant gratification. You can play HOI 4 at a faster speed and micromanage less. I suppose that my own prejudice comes from playing war board games. Invite a few friends over, play for 6 or 8 hours, leave the game set up, and play for 6 or 8 more hours the next day. I spent many a night until the wee hours playing The Rise and Decline of the Third Reich. Was an excellent game for its day.