@Sal Bro #173
but I'm (genuinely) asking whether it's reasonable/possible to be able to grow the quantity/diversity of crops needed to sustain the population of the entire planet on a veggie diet.
Well, since you genuinely asked :) As a veg diet is more efficient (uses less resources such as deforestation, water, grains) then I would have to say that is possible. The global impact of livestock is documented in the UN FAO Livestock's Long Shadow report. Press release here.
Whether people will want to or not is another thing. In my experience most people don't think about where their food comes from, or how it got there. Or, in some cases, just don't want to know, or when they do know, use such justifications as "it tastes good" or "our ancestors did it that way, so we should do it too." The "it tastes good" seems like a poor justification for killing anything; it seems so fussy and petty that just because eating the animal gives the temporary stimulation of pleasure, that a life has to be cut short? To some that is a good enough reason, but it is not to me. My desire to enjoy a meal is not sufficient enough for me to kill something, especially as there are available alternatives. In fact, the meat-substitutes are so good these days that I know of vegans that refuse to eat them because they are too meat-like, and they really don't like that.
As for ancestors, if you go back far enough our ancestors ate raw insects too, and further back, just bacteria. Our ancestors engaged in cannibalism, ritual sacrifices, slavery and other things that we now regard as morally questionable, and there will be other practices that are common place now that will be rejected in the future (such things as DADT, bullying etc) but it takes a long time for social norms to be recognised and accepted as things that need to change by a wider population. I can hope that the eating of meat will go the same way, but I suspect it is a vain hope, as there are too many personal preferences and business interests in the way. Still, I can advocate for it, just as much as omnivores can advocate against it. However, the animals that are slaughtered have no voice that can be heard, or listened to, and they have to most to personally lose.