This isn't even considering the fact that most of the prime bison territory is now farm or ranch land. So unless you're replacing cattle farms with bison farms

This isn't even considering the fact that most of the prime bison territory is now farm or ranch land. So unless you're replacing cattle farms with bison farms there's simply not enough room to sustain the sorts of populations that used to exist, with the possible exception being the huge swaths boreal forest of Canada. And it's debatable whether doing such a thing on a large scale even makes sense in the first place, because if you're just keeping the bison locked up in corrals they're probably going to be just as environmentally destructive as regular cattle.