Most egregious leap of faith: assuming most Americans are willing to give up meat consumption, or that we even should in the first place.
Beyond the production of meat, milk, wool, lard, and eggs, livestock has historically been vital to maintaining soil fertility by converting leaves, grasses and woody materials into incredibly valuable manure. You can't practice sustainable agriculture long-term without animal manure inputs, no matter how often you rotate your fields, plant green manures or nitrogen-fixing crops, or let the land lie fallow. Nothing beats the massive fertility deposit of a cow pie or pig plop. Farms that don't use apply livestock manures or practice rotational grazing are forced to utilize synthetic fertilizers made from natural gas, a decidedly non-sustainable practice.
Unfortunately, we've perverted the place of livestock in the past century through our massive increases in meat consumption and the creation of factory farms and feedlots. These atrocities are just as incompatible with a sustainable future as a livestock-free farming vision, and both must be rejected if we are to feed ourselves in a world ravaged by climate change in the 21st century.
A more realistic study would look at how much food we could generate if we went back to small-holding, heavily diversified farms like those common in the 18th and early 20th century. 40-160 acres, a herd of cattle or goats for milk and meat, flocks of free-range chickens, pastured pigs allowed to fatten up on acorns and chestnuts, fields of grains, alfalfa, and vegetables, orchards for fruit, woodlots for firewood and wild game hunting, streams, lakes and ponds for aquaculture and fishing, etc. Such diversity ensures maximum resilience against a changing climate, and makes sure the land remains fertile.
Basically, everything advocated in this famous and wonderful book: http://www.amazon.com/The-Self-Sufficient-Life-How-Live/dp/0756654505
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