I’d like to see a source for “what portion of feed they are given is also, largely, crop seconds or industrial byproduct”. The vast majority of information I have seen on this topic is that we produce more crops specifically to feed animals than we do to feed humans. Which, just from an energy perspective, is completely logical to me.
I don’t see how this supports your argument that eliminating livestock would not reduce land usage. 76% of soybean production is going to animal feed, do you really think that percentage would not reduce if you switched it over to providing food for humans?