...And what happens when we no longer need cows? It's not going to be happy fun time for the species, because as soon as there's not a reason to breed them then we'll have a few cows kicking around in zoos and that's it. Good for the environment, possibly, but bad for the species.
Do you think they are having fun now? They are bred to be eaten, if we don't need them we'll stop breeding them.
That's a very existential question that doesn't have an easy answer. Is a somewhat brief existence worse than no existence at all? If we eradicate fruit flies (or the many species that live short, brutish existences), have we done them a favor?
I am a huge fan of the Omnivore's Dilemma and recommend it wholeheartedly because while Pollan faces these serious questions, he doesn't simply resort to the easy answer.