The human body has been working with fat and sugar and such a lot longer than all the newer, more engineered stuff, making it seem even more reasonable.
I was reading Emma by Jane Austen recently which was written in the late 1800s and the story opens with the main father going on a tirade about cake being bad for you and nobody else caring. It is a scene that would fit all over the place today. It feels as though abundant, cheap and rich food has simply been scaled up to the level where the damage it does has become too visible to ignore anymore. It's not just a few lazy rich guys getting plump, it's the entire population.
Most people didn't have much access to sugar until the sugar islands were cultivated a few hundred years ago. Before that it was just fruit which was probably less sweet, has fiber in it and was more rare.
all the newer, more engineered stuff
Like what specifically?
The physiological effects of starch, which is composed only of glucose, are very different from the effects of sugar, half of which is fructose.
Until about two hundred years ago, all the sources of concentrated sugar, i.e. honey, cane sugar, dried fruits and boiled fruit juice were very expensive in comparison with other kinds of food and very few people could afford to eat them more frequently than a few times per year, at more important holidays.
Almost nobody could eat food with high content of sugar every day, like most people do now.
The availability of sugar or glucose-fructose mixtures that are cheaper than almost any other kind of food, so they are now added to reduce the cost of industrially-processed food, is a very recent phenomenon.
Another thing is that historically, malnutrition was a thing, not just something rare. The people would be getting serious diseases or be dying because of this or that missing in the food, there were whole epidemics of diseases that basically do not exist anymore.
The context here is that major food group that was pretty significant source of calories - complex carbohydrates was skipped creating dichotomy between sugar and fat.
The recommendations that I have seen are that up to around 50 g of sugar per day (25 g of fructose) should be fine.
Most fresh fruits have about 10% of sugar, so you need to eat more than a pound or a half of kg to reach levels of sugar intake that could be unhealthy.
Only a few fruits have more sugar, above 15%, e.g. grapes, fresh figs or fresh dates. Even with those you must eat an amount over what would satiate most people, in order to get too much sugar.
On the other hand, if you eat one chocolate (many of which contain 60% or more sugar) or a few candies you are likely to have already exceeded an acceptable daily intake.
And still, that doew not mean you nmed to cut complex carbohydrates and go keto ... especially if argument is how people 300 years ago eate. It was not keto.
And it is not used that way by people who push keto and low carbohydrates diets on HN.
And again, it renders fat vs sugar dichotomy meaningless.