Diets that require expenditure of willpower fail because the willpower expenditure indicates that something is going wrong with your body, which may well be shutting down or slowing down.

This is not true. Willpower expenditure is required for all sorts of decision making activities, so that a diet requires willpower expenditure is not evidence that it is damaging your body. A diet that requires significant willpower is severely suboptimal, which is why very few people report success with intense calorie counting and moderation, and why very many people report success with "trick" diets like Shangri-La, paleo, keto, intermittent fasting, etc.