As to the fatty liver disease, it's the sugar.
And I understand it does not sound reasonable, because humans had been consuming sugar, even in large quantities before 1983, so your conclusion on its face seems reasonable.
However, non alcoholic fatty liver disease isn't anything new in humans it's just new in children. It used to take until adulthood to develop the disease and now it's being developed as young as 1-2 years old. From the Liver Foundation, to the Cleveland clinic, to any pediatrician, the conclusion is all the same...it's sugar.
And for thousands of years humans have been purposefully over feeding animals for the sole purpose of causing fat livers. From the ancient Egyptians to modern foie gras, humans forcefeed these animals grains (sugars). There have been no mystery variables for thousands of years, feeding animals grains (sugars) leads to fatty livers.
As to humans it's the same thing, over consume sugar and you'll eventually develop fatty liver, it just didn't happen until adulthood until consumption of sugar exploded.
Still you are right, let's assume it's not sugar but some recently introduced variable, it is worth finding it out, but it's honestly not worth debating at the expense of kids health when we know whether or not there is some mystery variable, all that needs to be modified is sugar consumption and the disease would be eradicated. But IMO yes, kids used to eat less than some critical threshold of sugar, to claim otherwise is telling the Liver Foundation, Cleveland Clinic and American Diabetes Association they are wrong, it's even telling the sugar industry their accounting of sugar consumption is wrong. Humans have always gotten fatty liver from sugar, only the rate has increased and the age has decreased consistent with the increased sugar consumption. It really is on par with tobacco saying lung cancer or OCPD isn't caused by smoking but some other variable, and so we should find that variable rather than change our smoking habits.