By more productive, I mean something that is not a net negative contribution/actively harmful to our planet. If all farmers can do is convert subsidies into CO2 emissions for stuff that isn't technically needed, then we're better off with them not doing that. It's that simple.

This amount of corn is simply not needed and it never was. It only exists because of the subsidies. So, maybe grow stuff that is not contributing to destroying our planet and is actually worth something without a lot of subsidies. Organic farming seems pretty lucrative lately. Iowa has plenty of that happening already. There are plenty of things farmers can do that are actually needed that are much less harmful.