You didn't address, and I don't know: what is the relative environmental impact of growing human-grade soy vs. livestock-grade soy? Is there even necessarily a distinction?
As far as I know it's all just beans, they are all eatable by humans. The difference between resource consumption of different types of soy crops, if any, is not comparable to the difference between eating the soy directly or feeding it first to an animal, the second is a much more wasteful and environmentally impactful as you can imagine.
AFAIK there is no human-grade soy vs. livestock-grade soy. But the soy grown for livestock can usually be fed in the ratio of <soy for one animal>:<soy for many humans> (this ratio varies based on which animal and farming practice you consider).
You won't need to grow livestock-grade soy if you don't have livestock to feed. But we will always have humans to feed (because we're humans).