Pinning does more than beiging does is part of the problem but you are right that PnW is set up to make it so that folks are never entirely helpless - especially those with large city counts that can afford to throw soldiers endlessly. You are also right that there needs to be a mechanic that pulls folks out of the war. The trouble is that the only ways I could think of doing this would require rather large changes to the game, lots of tracking, and would impede the ability of midwar political maneuvering.