HFCS and “ultra processed foods” (food with all the fiber removed) are much more likely.
I’ve also noticed school lunches often jam artificial sweeteners into stuff that should not be sweet. They’re known to cause excessive hunger, and at the very least train kids that everything needs to be sweet.
There's some evidence HFCS is worse than sucrose (standard refined cane sugar) and some metabolic mechanisms to make that plausible, but it's a relatively minor difference compared to just the overall amount of consumption.
10 grams of "sugar" (sucrose) is 5 grams of fructose. 10 grams of high fructose corn syrup is 5.5 grams of fructose.
A diet going from 10 grams of any sugar per day to 50 or 100 grams of any sugar per day is going to have a drastically larger impact on health than if the 10 grams were sucrose vs HFCS.
30-40 years ago, sugar wasn't in everything, or at least not in the levels it is found today. People also chuck down liquid sugar like there's no tomorrow, which is also something "new" that wasn't around 30-40 years ago.
People today are less active, eat much more calories, many in the form of sugar, and many in the form of fat.
The main problem is, even if you wanted to go back to a "1970s diet", you probably couldn't today. Processed food is everywhere.
You may want to check the quantities.
Also, back in the day, not every product contained sugar or glucose syrup.