>If the grass pasture is already there in cycle, it's not being used for anything else. Might as well put cattle on it.

This is true, technically speaking. Practically, you couldn't produce even a fraction of the amount of meat that the market wants using this way. If all meat was grass fed meat, it would be a luxury good. I don't think that would be too bad to be honest, it would lead to much less meat consumption overall.

Basically, "just eat grass fed meat" = "everyone must eat a lot less meat, unless they're rich I guess"