I don't fully understand what you're asking. Could you clarify? Are you suggesting they might go extinct? Or something else?
(1). Is there any scientifically sound study proving that the quality of protein from plant sources is equivalent to or better than from dairy products?
(2). Is there any reproducible, quantitative data on leaving milk producing cows to fend for themselves vs caring for them in small -to- mid size "cow caring" setups (farms, one-cow ops etc.)? I ask this because both my maternal and paternal grandparents had 1 to 3 milk producing cows at any given point in time - from what I recall, they were well cared for (food, medicine, weather etc.)
(3). If we, as a species, were to stop consuming dairy products today, or cut consumption by, say 90% over the next year, what would become of the current cow population? How is that better than the current situation for the animals?
I am personally, utterly and deeply biased towards the survive-and-let-survive school of life --- so take what I say colored by my very, very, un-apologetically biased PoV.
You can look up Peter Singer and Yuval Noah Harari to start with and read what they have written about this. I mention them because they write well and mainly stick to scientifically supported ideas.
I haven't decided where I stand on the issues yet, but with a little research it has become apparent that you can have a very healthy diet without any protein form animals.
I've also discovered that more and more philosophers, ethicists, biologists etc, are starting to make the argument that our factory farming of animals may be the the worst crime in human history.
The modern “factory farming” is not something I care to defend for any intent or purpose. It may as well disappear tomorrow, as far as I am concerned.
However, I still see no argument against deeply caring domestication of a small number/herd of animals, providing them food, shelter and care in exchange for milk or unfertilized eggs. Can’t seem to find a convincing argument against that in my current ethics framework.