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.
