For reference, corn syrup is not in fact cheaper than cane sugar. But the US imposes a tariff on imported sugar that keeps the price at roughly 300% of what it would otherwise be.
US soda consumption roughly doubled from 1975 to 2000. And of course as you mentioned, the US switched from sugar to high fructose. The US Government poured gasoline on the fire, with its industry collusion re sugar and its wildly screwed up food pyramids encouraging the over-consumption of carbs.
US sugar consumption also oddly skyrocketed from roughly 1980 to 2000. It went from ~80 pounds to ~110 pounds annually per capita in just under 20 years (after being relatively stable in a 20lb range the prior 60 years minus WW2).
The era of sugar cereal, sugar breakfast, large containers of soda everywhere, a large junk food boom, a big pizza & carb boom (rapid growth of pizza chains everywhere), continued explosion of low quality fast food chains. And last but not least, a huge increase in sedentary lifestyles.
I suspect there are many reasons for increased obesity including more fast food, more (large) soda consumption, less physical play activity, etc. Simply blaming advertising, farm subsidies, or whatever may make people feel that they've identified an external bogeyman but it's not as simple as that.
Plenty of sugar beet here in Michigan. plugs nose