Most dairy farmers will slaughter cows after four years, or when their milk production begins to drop. Farmers want the most profitable animals possible and therefore don't wish to keep around cows that produce less milk. Dairy cows are usually sold to slaughterhouses and are ground up into hamburger meat at four years old, even though many can easily live longer than 20 years. If a farmer claims they keep all their older cows and/or bulls, they must have a tremendous amount of money and land, since keeping nonproductive animals is extremely costly, as any farm sanctuary can confirm.
Ultimately what it comes down to is the fact that cows don't produce milk for humans to drink; they make milk for their babies, just as every other mammal does. Dairy farming is inherently an abusive industry because it imprisons animals and forcibly takes what does not belong to us.
Drinking cows' milk is also a rather perverse act when thought about. Drinking the lactation of another species is strange enough. But to drink the milk of a 1,000 lb, non-primate animal, completely covered in hair and who eats grass, is even stranger. Any claim that drinking cows' milk is "natural" should be met with great suspicion.
