a. Surely you see the very exact parallel?
b. Darwin onwards, evolution has been understood to be natural selection, what we have done and continue to do with both plants and in particular livestock would not come under that. These genes that are being successfully passed on in modern chickens/turkeys etc. wouldn't let them survive and compete in the wild beyond a few years (for example modern chickens routinely collapse under their own weight within 2 months)
Also, you say our purpose is to pass on genes..you're ascribing a purpose to a natural process. You're also condemning childless humans as purposeless when in fact as you yourself said population decline will be necessary to preserve the environment.
I don't really find a film to be pertinent parallel in such a debate. But when I said I wasn't bothered I meant that it is not something I fear or care about, hypothetically speaking.
Exactly though, cattle is ill equipped to survive without being part of the industrial food chain or by being some sort of hobby breeding endeavour for a human. To stop eating cattle would condemn it to extinction most likely. That might satisfy your moral needs but I don't think the cattle would be too pleased about it if they were able to communicate. What exactly is your vision for cattle beyond it's highly successful evolutionary trade off with humans by being part of our food chain?
But yes, evolutionary biology does ascribe a fundamental purpose to that natural process, Dawkins terms living beings as 'survival machines' for genes. Genes are considered the fundamental unit of evolution as they are the only biological entity that are stable in evolutionary time, you, for example, if you procreate, your children will only be half of your genetic makeup and those genetics will continue to be more and more diffuse with each generation, whereas genes can survive in exact copies for a hundred thousand years. The theory also suggests that our qualitatively different relationship with our blood relatives is founded in our common interest that is the propagation of our genes. Your siblings from the same parents are the only people in the world with who your share 100% of your DNA.
Humans have gone beyond those fundamentals philosophically and can find purpose beyond that, but why do you think so many people get so hung up on having children or why it is seen as almost obligatory in all human societies?
As a slight aside, let's say that people are outraged by the death of this Lion because big cats are so rare, large and beautiful that we ascribe a higher value on their life. You say 'what about killing cows they feel pain too'. Well what about that snail I stood on this morning by accident or those ants that I poison in my kitchen and what about the research that suggests that plants feel pain too. Where is it reasonable to draw a line philosopically?
I accept arguments that say eating meat is problematic for the environment but that is a case of volume. That said, I am yet to hear a convincing argument for the inherent immorality of eating meat, however. Not that I don't think it can't be a positive lifestyle choice.