But your basic point still stands. The Montana native grasslands can only support a few cattle per square mile, so it definitely does not work at scale.
This should not be a veganism vs organic meat discussion, but an organic meat vs intensive meat production discussion. And a separate sustainable crops vs soil-depleting crop production discussion.
You can't feed everybody on just meat; you're going to need crops of various sorts. Feeding crops to animals for meat is incredibly wasteful, but having animals graze on land that's otherwise unusable, is a great idea. I hope this is the future of meat production. But it's not a replacement for sustainable crops.
One could readily argue that most soils are better used for sustainable grazing. Tillage is very intrusive, destructive and unnatural. Most food crops require tillage.
I used Montana as an example of a soil that would be quickly destroyed with a little bit of tillage. Luckily most soil isn't as fragile as Montana's, but virtually all soil is damaged by tillage.
It's all moot though -- our current level of meat consumption is way above the level that can be met through sustainable grazing.
What matters is how you can produce food efficiently and sustainably. There is soil that can produce more food in a sustainable manner through crops than through grazing. There is also soil where the opposite is true.
One of the key goals they set out, aside from getting people to rely less on livestock agriculture in the first place, is to try and convert more of it to CAFOs. Yeah, factory farms are ugly, but they found that it's much more efficient from a land and energy use perspective. As have other disinterested parties. The loudest voices saying otherwise seem to be ones that have a financial interest in selling more grass-fed beef.
A subtext here is that we live in a very different era than the one that the one invoked by more nostalgic approaches to agriculture. Once upon a time, people had no choice but to figure out how to feed themselves off the land they occupied, and nature put much firmer limits on how many people could occupy certain areas. Nowadays, not only is getting food shipped in a realistic option, sometimes it's even the more economical and sustainable one.
Back to Bison? Bison are much more effective at preventing forests from forming than cattle are. So "natural", but from an ecosystem protection and climate change perspective probably not the best choice. "What's best for the environment" is often a large set of conflicting goals...
This is a topic that I've just started looking into on my parent's small 20 acre farm, which run a few head of cattle and due to drought, has caused the soil to be highly compacted. Thankfully, the farm itself used to be an orchard (oranges and kiwifruit mainly), so the cattle have alternative feed sources beside just grazing land. (Believe it or not, cows love fruit, including the leaves!)
Recently I attended a seminar run by local government in conjunction with a group [0] about cover crops (which admittedly is more about vegetable farming, but parts can be used for grazing too). Their focus was mainly about getting the root structures right, which the above article hints. The link below shows a rather wide range of farms and environments that are being tested on. It also has information about 'biopesticides' [1], ie. plants that will help mitigate/eradicate unwanted pests, without need for pesticides.
[0] http://www.soilwealth.com.au/demo-sites/ [1] http://www.soilwealth.com.au/resources/fact-sheets/pest-and-...
Mixed feelings, I suppose.