Sure, that land could go to growing additional corn or potatoes, but there is in fact other land that could go to that, but doesn't, already. More people could grow more potatoes and absolutely saturate the potato market, but it wouldn't necessarily be sustainable to do that. The amount of potatoes we grow isn't constrained by the amount of land we have, it's constrained by the fact that people need to make a net profit off growing potatoes, so the amount we grow is a set ratio based on how much it costs to grow, process and distribute relative to the amount of demand that exists. Prices will fall and margins will go negative if they over-produce potatoes, and people will be pushed out of the market, meaning production will fall again, which is why that doesn't really happen already. If you want to make foodstuffs from potatoes more abundant, you don't need more land you need to streamline the production so that it's cheaper. higher demand for potato-growing land will follow from that, not the other way around.
If they can't do the feedlot thing so they don't want to grow the stuff you mentioned, there's no reason to expect them to switch to the corn, potatoes etc already unless there's some economic incentive to do that, and you've just massively reduced the value of their land, by definition, since you're preventing them from growing the thing that's the most economically viable out of all their options. The effect of doing so would be like the logic behind tariffs. i.e. bad logic. There's also the problem that really cheap food floods into poor countries as imports and this is in fact a big cause of third-world unemployment and collapse of rural economies, since you cannot really compete with international agribusiness. Just producing more food isn't necessarily a solution, so saying that we could switch those feedlots to producing more potatoes, corn, wheat and what have you isn't necessarily going to be a good thing.