Sure, i understand the McDonald's argument--and it is the same as the comparison between responsible crop farming and corporate, bottom-line crop farming. But you know, a lot of people in the country know where their food comes from, even if they don't producve it. And, in Ontario, at least (and i suspect many places in the U.S.) it is a selling point that the meat offered for did not
come from a feed lot. One supermarket chain even offers meat in a package with a photo of the farming couple on the label, with something to the effect of "John and Jane Doe farm near Woodstock, Ontario." There's an entire high end chain of meat outlets that makes grazed beef a selling point.
Yes, i specified grazed livestock--but i'd not be surprised to find that the proportion of responsibly grown soy beans is roughly equivalent to the proportion of grazed livestock--i.e., in both cases a fraction of total production. But you still have problems you can't avoid with crop farming--the fossil fuel usage, the cost of the farm implements, the use of fertilizers, pesticides and herbicides. Those things simply aren't there with grazed livestock.
But this isn't an either or proposition--i'm not an exclusive carnivore, i'm an omnivore. And it isn't necessary to choose either meat-eating or tofu, and the middle be damned. It would quite responsible and make good sense in terms of energy and resource efficiency to graze steers in a field one year, then plant alfalfa there the next year, both of which processes fertilize the field (alfalfa is a nitogen-fixing plant, if i recall correctly), and then put crops in the following year, creating a cycle which reduces the dependence on chemical farming. Actually the problem with feedlots and what you call "dirty" crop farming is that it is corporate, bottom line farming.
Your argument, then, isn't really an argument for soy-farming over cattle farming. It's an argument for sustainable agriculture vs.---what? exploitative agriculture?
Soy farming isn't intrinsically unsustainable any more than cattle farming is.
Well, it isn't "unsustainable" if it is part of a crop rotation farming plan. But it will exhaust the soil pretty quickly in monocultures, and monoculture is what corporate farming is usually about.
My problem is with two attitudes: the first, the hypocrisy of saying my way of eating is superior, and yours should not be allowed, which is the attitude of a lot of vegans. The other is the either/or, throw the baby out with the bath water attitude. Farmers are perfectly capable, and have a thousands of years tradition of cropping and grazing livestock. I see no reason to ignore the benefits of grazing combined with crop rotation to produce healthy, sustainable farms. I see no reason to give up eating greasy, juicy, wonderful meat just because a pack of strident vegans (present company probably excepted) howl for the blood of the evil carnivores.