I’m gonna guess that wild grazers get predated on by a much more varied species than just carbon polluting humans and their domesticated wolfs. In the wild a bison might get eaten by a pack of wolfs or it might live long and die of old age and be eaten by a host of different scavenges, as well as insects and fungi. All supplying other niches in the ecosystem. I’m guessing that in the wild the methane that gets emitted by grazing herd animals is offset somewhere else in the ecosystem by the niches which these animals create by their behavior.

It might be that allocating all of these grazing animals for just one species destroys most other parts of ecosystems which would otherwise have adjusted for the greenhouse gasses emitted by them.

Or who knows, maybe we fluctuate between warm periods in our ice ages because enough grazing animals have farted enough to start a runaway global warming effects... though I doubt it.