> You're targeting something that is three levels of abstraction away from what you actually care about. My position is that it makes more sense to target what you're actually after: calories.
It's not abstracted away as healthy eating is concerned. Overconsumption is downstream.
You haven't elucidated how you'd merely target calories through policy, but leaving that aside, a) by default people do not count calories nor would they as a measure to protect against weight-gain, b) it's redundant given a whole-foods diet, no one becomes obese from too much broccoli, chicken breast and lentils, c) for those looking to lose weight, mere calorie counting absent leveraging satiating foods and eschewing junk is woefully ineffective in practice, because of lack of sustainability. Dieters typically do lose some weight, then gain it back. Not only is it difficult to adhere to, it's difficult to eyeball calories on a plate, particularly when they're processed foods, such that they'd have to weigh everything on a scale indefinitely.
Encouraging healthier eating patterns solves several problems at once. It protects against overconsumption, and against disease, which would lessen a burden on the healthcare system. That seems quite pragmatic to me. What's at stake is certain corporations stand to make less money, and corn farmers sell less.
Whether through change in diet patterns or "just eating less" as you might posit, if on the national scale people did end up consuming fewer calories and lose weight, then they'd more than likely consume less sugar/HFCS. The end result is still that a healthier populace == selling less corn. We can't discount any and all policy on the conceit that inconveniencing corn farmers is not acceptable.