It's a heuristic, a rule of thumb. Think Michael Pollan's grandparents if you like (he's 65).
The change in American diet was not subtle; it was huge. It was due to what's cheaper / more profitable for a huge industry to produce.
It's similar to what's going on right now with the software industry -- cloud services with mobile clients is great economically and great for collecting data, but not necessarily in the short-term or long-term interests of the users.
If you believe in capitalism, you would have to explain how optimizing for cheap and superficially appealing food would NOT happen.
Who knows what additives were in our grandparents' foods.