I totally recognise this. In this example, and the comment below, you measure in unit of nutrition (which I agree is the important factor), but no one has disputed that a human diet from plants compared to meat would not require more usage of arable land. Humans require a larger variety of food than livestock does, so you can't just use efficient (efficient as in terms of nutrition per hectare) plants for humans.

My concern is that we don't have enough land for this, and the fear/threat of overpopulation is already upon us. And has been for decades.

Are you really thinking this through?

Consider that all of the land currently used for livestock were converted. You then _also_ have all of the land used for feedstock for the livestock.

And then there's the fact that most diets globally are predominantly vegetable based anyway - meat is a rare addition.

Land used for livestock != Land used for plants.

You can't convert it, or it's already converted. Usually for corn.