I think the degradation of quality in certain brands is like a frog boiling; it's been going on for years, steadily being watered down so that from month to month or year to year, there isn't much difference, but over the course of a few years the total difference becomes quite large. Another possibility is just that they're slowly adding more preservatives or something that subtly alters the taste profile because they expect for their candies and things to stay on shelves longer, or something like that? Either way, the change is so gradual most people probably don't really recognize them changing at all. Their existence, like with most things, is almost certainly down to name recognition. Another thing to think about is that most candy and food brands are owned by mega conglomerates that own a bunch of other brands too, for instance Pespi Co. indirectly owns Taco Bell, KFC, and Pizza Hut through another company that they spun off called "Yum! Brands" So even if they have one product that is underselling, they can still maintain shelf presence through the sale of other things.
Narcotics, sugary products, caffeine products, anything which isn't water and bread are targeted at a group of addicts who buy a dozen times of the amount a stasticially normal person would get.
Other than US entertainment, I don't think so. I don't live in the US though. The quality of food has been going up in many countries since people are realizing(beyond the moral beliefs of the past and with scientific reasoning) that sugar, alcohol and meat are dangerous to have constantly.
I think as a kid we liked sugar much more than as adults. Junk food isn't as nice and our bodies are saying that eating this a lot will end up giving you gout.
I tried Cheetos for the first time recently after they were released where I live. They were absolutely disgusting. I have no clue how Americans eat those things.