Yeah, but they are not as irrational as you are and producing milk costs money. If there's no market, they will stop because they are not fuckin lunatics and they don't have infinite resources
No, you're just making no sensical argument at all. Milk was farmed from dairy cattle because it was consumed by humans. It's simple supply and demand. There is no rational argument at all that if mankind would stop consuming milk, it would still be farmed. Why would any farmer go through the effort to upkeep cows and keep them impregnated to make them produce milk if they cannot trade it or won't consume it? Yes, humans have free will but they don't produce stuff with high effort just for fun. Except maybe very sick minds that just enjoy the animal cruelty. And you won't elaborate what your actual point is anyway.
Also, not that it matters but you're arguing that dairy farming existed before the market is simply wrong. There has been trade between human civilizations long before we began domesticating animals.