but even in those diets, carb elimination is in the first few weeks and then you incorporate healthy carbs back slowly, now if someone is just going to yoyo by losing the same 10 lbs of water weight every 6 months then that is a diff story.
at their core long term restricted car diets actually restrict your total calories mostly be reducing empty calories and that is why there can be long term sustained weight loss, as far as a lifestyle choice, decreasing sugars and simple carbs should not be that hard, note i said decrease not eliminate.