We really need multiple words for diet.

It means at least two different things:

It can refer to specific planned period of eating a certain way to achieve a certain end.

e.g. Atkins diet(please look up how he died before considering this), fodmap diet , low GI diet, potato diet

It can also refer to the normal way a particular person at particular place and time in history eats.

e.g The Roman diet, Celtic diet, American Diet, French diet, Pandas diet, Neanderthal diet ,Vegan/Vegetarian diets, Kosher/Halal diets

I would imagine extreme diets are for viable for those that require extreme weight lose. Someone looking to lose 100+lbs would be able to use it to lose enough weight to become active again and start to make lifestyle changes that will support long term weight management. People looking to drop 10lbs though are very likely to gain it back once they start eating like a normal human again.