You wrote - "It's not that it's wrong to eat a cow - that's a cow's purpose."
According to whom? I for one believe that it is VERY wrong to eat a cow, and that cows (along with other species) have a purpose that is distinct from a human's use for them. I didn't create the cow, so I don't claim to know its purpose. I also believe that all animals - including cows, pigs, chickens, and dogs - have souls. If you believe, as I do, that animals have souls, how can you eat it? I can't.
Sometimes "just say no" DOES work, and it is the only thing that does.