Not OP, but land use due to animal feed is relevant to mono-culture because reducing land use for animal feed would free that land up for other uses (besides soy beans), either for other crops or to leave the land unutilized and left to nature.
The land would not be left unutilized or left to nature, and those same practices are used to produce all of your food. Take a drive down the central valley of California and the vast lettuce and fruit farms will have less genetic diversity than your backyard garden. Monoculture production is a consequence of efforts to increase production efficiency, regardless of the actual crop being produced.