A few reasons: 1) again, it's partly a national security issue. Under crisis, "global supply" is a concern; just ask Germany after trying to turn away from Russian fuel supply 2) Infrastructure has a relatively large lead time; we can't just ramp up production on a whim. 3) It's odd that you point to global supply as the rationale while simultaneously advocating the largest global supplier severely reduce production. Again, that feels like circular logic. Ie "The US doesn't need to produce ethanol because the world has so much ethanol production." No, the world has so much ethanol production because the US produces a disproportionate amount. Remove the latter and the argument doesn't hold.
I don't think we disagree that making food more expensive can change eating habits. I think we disagree on the most effective vehicle for that.
Look at it this way: we both seem to agree that calories are the problem. Your argument hinges on sweeteners being a proxy for calories, and HFCS being a proxy for sweeteners, and agricultural corn being a proxy for HFCS. 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.
If your stance is getting rid of corn subsidies is administratively simple compared to targeting calories, I think I disagree mainly because of the administrative burden of all the other effects we've discussed.
I don't disagree that deterrence is part of an overall strategy. I'm simply pointing out that one should be wary of the tradeoffs. Policy is about prioritizing, and IMO there are likely more pragmatic approaches with less tradeoffs that need to be managed.