Note that this is only true for modern agricultural practices. The norm for much of human history was to graze cows and sheep on grass, which humans can't consume at all, and which has evolved in concert with grazing animals. Pigs would often be fed on leftovers and also plenty of non-edible foodstuffs. Chicken were more of an exception, but then again their eggs were eaten much more often than sacrificing the chicken itself.

There is nothing preventing us from going back to more traditional agricultural practices in this area, just as we need to do for most crop harvests to avoid depleting the soils everywhere. Of course, this will require meat to become more expensive, seasonal, and a less common food overall. But there is no reason a 0-emission world would require the whole humam population to be vegan.