I don't think anyone's suggesting doing without animals - just that the industrial farming of animals is totally unsustainable. Animal farming used to be done by grazing on marginal land or, as you point out, by using marginal products as feed, as opposed to using prime land to intensively produce cattle feed.

When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes