1. Before we kept cattle, there were large ruminants over much of the planet - bison, buffalo, deer, elk, reindeer - which will have the same impact as grass fed cattle
- They lived in naturally-limited populations fitting with their environments, unlike cows, which are mass populations on artificial, human-limited populations. Hence, devastating habitat destruction.
2. It isn't possible to ignore crop quality - if people won't or can't eat it, then it is pointless, plus as said, waste products from human food also go to animals - it is already established those are bits we can't eat.
- Protein is protein.
And we need less than people realise.
3. Both being an omnivore or a vegetarian includes drinking milk and eating eggs - for that you have to breed animals and birds and you have a surplus of males, which you either mature and then eat, or kill at birth. At present in Western farming practice most dairy cattle bulls and cockerels are killed just after birth. Commercial laying chickens are "autosexing" which means the cockerel and hen chicks are visually different, so people have the lovely job of separating the sexes and killing the cockerels. There are also a lot of bereft cows who give birth then immediately lose their calf. If the calves were grown to the point they are weaned and the mother is moving on, that would be more ethical. However, as mentioned earlier, dairy cattle are not "beefy" so they are less popular for eating, though there are people who are raising dairy calves for meat - or putting a beef bull on a dairy cow, so the calf has meat value. Though breeding pure dairy calves is still necessary for continuing the breed, with of course, the production of surplus dairy bull calves.
The detail is important in all of this - especially in terms of ethics.