Les, it would be more practical for all omnivores to eat less meat (everyone in the west eats much more than they need simply because they can afford to) and more veg. There would be a lot of health benefits as well as environmental ones.

If huge numbers of people turned veggie, the there could be sustainability problems as protein rich legume crops do not grow well in cooler climates (and sheep and cows do).

I'll stick to the luxury of my veggie diet though, if you don't mind.