The argument for reducing meat production isn't about eliminating pastoral settings. Some people want to eliminate 100% of meat consumption, but I don't think that's entirely necessary. Eliminating factory farming is necessary though, and the methane produced by that method is entirely unsustainable.

Also, if you're only eating grain, then yeah it would take a lot of it to meet nutritional requirements. But if you're eating grains and legumes, then it's much easier to ensure complete nutrition without any meat products.

It takes 25kg of grain to produce 1 kg of beef. If the land used to produce that grain were instead used to produce grains and legumes for human consumption, it would produce more than enough to end world hunger