I really dont like that argument (even if we change "designed" to "evolved").

It makes it sound like species evolve in isolation without any interaction, influence and interdependencies.

Cow milk in its current fit-for-human-consumption state wouldnt exist without us and has not been "designed for the calf" - it's been designed for humans by humans (™), using cows.

Or to move it into nature - Couldnt we argue that if a predator hunts a prey because it's an efficient use of its time due to its nourishing milk, only those who produce the most nourishing milk will see their offspring survive (since weaker offspring is easy prey) -> Milk is most nourishing because another animal benefits from it too.

The milk is still "intended" for the offspring, sure, but I take issue with it as an isolated argument.

That is of course not an excuse for the sometimes indiscriminate cruelty involved in it's production, I mostly agree with vegan philosophy (I'm not one, but my meat consumption is kept to a tasty minimum).