Im sorry, but what kind of mammal biology are you talking about that says that two different types of milk come from the same animal?

Also, milk production usually does go down somewhat over time and sometimes even does cease, and it’s different for every individual, which makes it less predictable and less profitable to just impregnate a cow once. This is why dairy farmers almost always try to impregnate their cows yearly. Here’s one source, there’s plenty more coming up if you look it up on a search engine though https://www.rspca.org.uk/adviceandwelfare/farm/dairy/farming