The claim here is variously:
While it is theoretically possible in some alternate universe that we could be growing free-range meat on soil that otherwise is unfit for growing crops, that just isn't the case in our actual reality. 80% of the soy we grow goes towards feeding animals. The US alone could feed 800 million people with just our food supply. The sole reason for worldwide hunger is that food is fed to livestock while people suffer from hunger.
That involves radically reducing meat and animal product consumption. You're looking at completely ditching non-pasture-raised animals, which is more than two-thirds of the US diet, ballpark numbers. So on top of that, you want to live off of 1/4 of the current beef supply. So again, the big-brain carnist solution is to avoid going 100% vegan by going 92% vegan, which I guess I'll accept if you want to help resolve the sustainability issues. But who here is going to go from eating meat every day to once every two weeks?
Besides issues of overgrazing and destruction of natural habitats that would result from a switch to an all-grass-fed beef supply, substantial greenhouse gas emissions would still remain.
It's impossible to get past the fact that it isn't ethical to kill something that doesn't want to die, and so there's no way to skirt that issue by claiming that "it's just so economical to hang animals upside down as they panic and put bolts through their heads and slit their throats". That is just not a response that makes any sense in any other context. We wouldn't accept the economy as a reason to keep slavery going, and we therefore shouldn't accept it for slaughtering animals.
Right now, if you go to a restaurant or get beef at a grocery store, it was created in a world where the beef was not all pasture fed. So regardless of what is true in a hypothetical universe, eating meat still hurts people and the environment just the same, right now, today.