If this ever became economically viable (and mass socially acceptable) it would cause the virtual extinction of farmed animals as we know them. (Chickens, cows and pigs enjoy vast population numbers, even if poor lifestyle, due to human consumption). If we no longer needed them, what does PETA think we should do, release them to the wild they are no longer capable of living in after millennia of domestication?

Not necessarily true, adaptations to consumption of milk has happened within a relatively short timespan: http://www.scientificamerican.com/article.cfm?id=neolithic-europeans-lactose-tolerance People live much longer than they did tens of thousands of years ago for several reasons, one among them being diets were no where near as varied and able to supply all the necessary nutrients we require as we have today ("Mammoth steaks AGAIN?"), even though there is plenty of unhealthy modern food, there is plenty which has been created within the even the last 100 years which has improved the quality of healthy eating. There have been plenty of developments in the history of humankind which have provided advantages, you really can't say that suddenly stopped 10's of thousands of years ago.