I'm saying that in some cultures that had food issues, and may still have food issues, that which can be used and consumed will be used and consumed. Wasn't that how why got dogs in the first place? Protection, hunting, work, and in a pinch, rations. It's only when you suddenly have abundance that you can start being subjectively selective. When you then start trumping up artificial boundary conditions on what is acceptable and not, you have absolutely no place to admonish those whose culture didn't suddenly decide that it's not ethically edible but instead selected other things as their 'sacred cow'. Especially when those convictions are ONLY because you can afford not to, and will be sacrificed the moment it's not convenient.
Don't often hear about sea kittens at a coastal city, or farming vegans...it's more young urbanites for whom the only exposure to an animal is the family pet, and their 'hunting gathering' is limited to supermarket sales.
Buuut if you want to believe we can turn all of the poorer parts of asian, south american, africa, and even our own less affluent areas into pill-supplement-popping balanced vegan dieters...by all means continue to think that. Personally I see no real difference in eating horse, cow, pig, whale, dog or rabbit (that said, I don't eat any of the previous). Now if you want to argue ethics in raising said creatures, methods to kill, etc...then yes, I'll say we have room for improvement there (vastly in some cases).
As for the biology lesson, I'd imagine we'd have evolved a different dental structure, a different digestive system (one more optimised) and wouldn't have to actually plan out our diet so exotically if we were meant to be plant-only. No other animal that I'm aware of would need a laundry list of things to stay healthy on a vegan diet. Herbivores could continually graze on local flora, carnivores opportunistically hunt local fauna. Guess that leaves omnivores for us...