Well, if people start eating only plants, then the’re will be less plants in the environment, not only for humans but animals too, and a lot of animals need plants to survive.
And not saying that killing animals is okay, but in reality most people take care of animals because through them they’ll have food to eat, and mostly dairy. They make sure to keep them healthy, and for that they’ll be able to reproduce healthy.
Not just food that animals are able to provide, but also help human with living. 🙂