In a small dairy situation the calves are kept off of the cows for either the morning or the evening milking and so the cows' milk production remains high the entire time. The calf weans, but from the cow's udder's perspective the calf is still suckling like mad and so the cow keeps producing milk. The calf grows up and is bred themselves, doubling the milk output over the course of two to three years.

Now this isn't sustainable in a paperclip-optimizer kind of way, so only the best milkers are kept and the ones that either have a hard time getting pregnant, or are ornery, or produce less milk, or get sick regularly wind up getting harvested for their meat. But there's no need to kill the calves (at the local dairy scale).