A grass fed cow by definition will not use any extra calories from feed crops. At least in the United States, there is a fair bit of land that is both non-arab

A grass fed cow by definition will not use any extra calories from feed crops. At least in the United States, there is a fair bit of land that is both non-arable and yet perfectly sustainable for cattle (much of this land was historically home of a bovine species, the plains bison, in the first place). I'm thinking areas like certain areas of Texas, Wyoming, Montana, etc. There are probably other areas in this regard, so I think it's possible to keep some meat and actually have it be relatively sustainable. Everyone's overall point is correct, however, in that this does not apply to a lot of meat production today; keeping only the more sustainable practices would end up making meat a luxury product (as it was in earlier times).