The problem is that the levels of sweetness, acidity, salinity, carbonation etc. in diet soda have become normalized. People give these dessert drinks to children, and drink it like it's water.
You know how overpoweringly strong some chocolate mousse can be? The kind where you can't finish a tiny slice of cake because more than a tiny sliver on your fork is too rich? Or how some foreign cuisines can be so potently dosed with curry or pepper that you can't taste any other flavors in the dish? Imagine replacing white bread throughout your diet with that chocolate cake. Or the punch of salt and pepper on an egg with that level of curry.
Ask someone from 100 years ago to sweeten a glass of lemonade "to taste" and you'd get something so weak that a consumer of diet soda would mock it like the meme "hint of hint of lime" or "transported on a truck near strawberries" flavors of LaCroix.
Yes it's better that they drink diet than regular soda. No, it's not good to normalize that flavor. I think human appetites are just not set up to handle some stimuli that previous levels of foraging or farming, chemistry, and distribution systems could not create.
