My argument on marginal land is prima facie so far. I picked it because it seems obviously true on the surface, so I can let you provide your points to try to blow it up. I'm referring to the land use problem, which is the often-cited vegan argument that livestock land could be instead used as forests or croplands to sequester carbon.
If you want to contest the 2/3 marginal land number, I'll cite a few references, but it seems an odd number to consider "magic"