First of all, I never said that dieting will kill you, I said that if you're always on a diet that actually works, you will continually lose weight until you become anemic and underweight. "Dieting" means managing your food intake in such a way that you lose weight, you're supposed to stop dieting after hitting your goal weight and instead switch to a life-and food style that keeps you at that weight without gaining or losing weight in general.
No one has claimed it's impossible to change your weight, the issue raised was that diets are usually bullshit because they try to sell you the idea that "just do this for a few weeks and you're done" when in reality, you then have to also change your entire lifestyle permanently to one that can maintain you at that weight. Going back to what you were doing before dieting will just get you overweight and in need of dieting all over again, which conveniently means you're back to giving money to the people trying to sell you dieting bogus.
you seem to have the wrong idea about diets ,if you´re talking about diets from magazines and shitty internet articles you´re probably right but If you go to nutritionist or a doctor they will tell you that diets usually involve eating heathier and at proper times not eating less
A lot of dieting manga have this flaw, where they will introduce a diet that works for the character but then once they hit their weight goal they just stop and go back to the same behaviour as before, only conveniently they don't gain the weight back because fiction doesn't have to follow rules. Not saying all dieting manga do it, just that it's usually a good sign that the author is writing about something they probably don't really know much about.
well to be fair most of the times when characters gain weight is because they started overeating and/or having an unhealthy life style and when they get back to their original weight is implied that they also went back to a somewhat healthy life style