The more important point the comment you're replying to makes is not "what if people could diet and exercise" - i.e. accept the modern American lifestyle as given, plus force yourself to go to the gym and eat chicken and broccoli - but rather that the modern American lifestyle in fundamentally structured to lead to people being overweight.

Instead of being forced to drive everywhere for the most basic possible human needs - like getting groceries, going to the doctor, or dropping kids off at school - as is the case in 90% of America - what if you could walk to those places instead? You would get exercise as part of your every day life, with no extra effort!

What if instead of corn syrup being so heavily subsidized, we could use more filling sweeteners in a lower amount instead? What if people lived closer to agriculture, instead of in faraway suburban tract housing only accessible by car, so they had easier access to fresh meat and vegetables, instead of ultraprocessed package food?

These dreams are not "diet and exercise", they are a fundamental reshaping of American lifestyle that would directly lead to weight loss. We know this, because America used to look like this before, say, 1940. In old photos you see people in huge crowds in streets as they walked to their everyday errands, and menus and recipes of the era are mostly minimally processed food that is mostly local. Americans of the past were not overweight, because the way society arranged its physical existence didn't permit it!