It's not just about soy. We're talking about switiching to a whole grain diet rich in vegetables and fruits, nuts, legumes, seeds, etc. Not overnight, of course. Would there even be a chance of that happening? No. But, yes, it is practical. In fact, it's been estimated that if we took the grain we give to cows and other livestock and gave it to people, we could feed the entire world with much left over. By virtually all accounts, eating food derived from animals is wasteful.
The senseless waste of the world's growing meat-centered diet is illustrated by a statement put forth by the Population Reference Bureau: "If everyone adopted a vegetarian diet and no food were wasted, current food production would theoretically feed 10 billion people, more than the projected population for the year 2050."