This implies that the land used to grow cow feed couldn't be used to instead grow human-consumable crops or left fallow as a carbon sink. I'm not aware of any studies showing that.
Deforesting the amazon to grow corn for cows is something I'm sure we can all agree is bad.
Should we use livestock to eat cover crops and waste products like corn husks and cobs? This way we reuse waste and produce high quality protein to supplement our diets.
Should we graze cattle on natural grasslands like the great plains? Especially in rotation with human foods, like the cover crops mentioned before?
So sure, go for 100%-grass-and-not-human-edible-food-fed beef, but that's not the current system.