It depends on one's reasons for vegetarianism. Personally I'm vegetarian because it prevents suffering. I don't value a species, I value individuals. A species is just a categorization that cannot feel pain or pleasure. We can imagine a continuous line-up of daughter, mother, grandmother and so on, up to the point of the last common ancestor of humans and cows, and then forwards in time again to modern cows. Within that line-up, there would be thousands of species, and virtually all of them went extinct already. A common definition of "species" is that groups of animals belong to different species if they cannot have fertile offspring together. I don't see how this is a relevant criterion for awarding moral concern to species rather than individuals. And as for the individual cows, yes, they would die eventually (and then we might as well eat them), but so they would if we keep breeding more cows for food-purposes, so I don't quite see the point.