I don't believe that the American lifestyle and obesity epidemic are inevitable or hopeless. The common lifestyle can change for the better, and it wouldn't necessarily "tank" the economy any more than automobiles replacing horse-drawn carriages did (although there were folks warning that it would). As some businesses went out of business, other businesses would take their places.
We do have some nation-wide health-improvement trends, but they're the minority (right now), but the minority can become the majority. Of course change has to be triggered by individuals (most majorities started as minorities).
Right now, not everyone can prepare food using fresh ingredients - but most people can improve their current habits. The more people who make the change, the more popular the change will be and the more people will join "the movement."
None of us can change the world ourselves, but we can make changes for ourselves. We can get excited about our changes and tell other people and hopefully get a few more excited about it too.
When I was growing up in the 70's, you were lucky to find three varieties of apple in any grocery store. You were lucky to find more than six or seven types of fruit (not to mention varieties of each fruit) at any one time.
Changing our food and lifestyle habits as a country, wouldn't be an easy or a quick change, but I don't think it's an impossible one, but it will take people who believe change is possible and who are willing to change whether or not anyone else follows.