they would die ... if they dont get milked
Your question is a really important matter in the vegan lifestyle, thanks to asking me that.
Every cow as you know is a mammal, just like humans, that means that a cow don't produce milk for free but she needs to get pregnant.
Now in nature a pregnant cow will give birth her calf, that will milk her during the weaning period that usually last around 4 to 6 months.
In this situation the cow won't die, because she will get milked by her baby. It's how nature works.
But the process of milk production made by human is different and really dangerous for the cow and calf. When a cow is pregnant, (and she get pregnant artificially) after the calf birth,
it will be took apart from its mother, to permit milking the mother until she's not too old to get milked, then she get slaughtered.
As a vegan you won't tolerate any kind of animal exploitation.