That doesn't mean the AI should let it happen to the player. The AI should be free to act, not constrained, but those constraints aren't a lack of sandbox, what they are happens to be a symptom of TOO MUCH sandbox in the game, because the player shouldn't have that kind of free reign anyways; realistically, their country would be strained so much at that point, or would have fallen apart.
Paradox seems to always forget that HOI has a focus on a war, not a long time period, such as EU, which has many wars. I hope they can remember this and not go in the path which some people would have, where it's EU for a few years in modern times.