Milk was a valuable resource that helped deal with childhood malnutrition in the post war years when other sources of nutrients were scarce.
It’s high in calories, vitamins and fats, so was perfect at the time.
For the majority of us, milk is unnecessary. The environmental damage done by the dairy industry is unnecessary, as is the death of millions of cows.
Do we need to cut out milk altogether? Probably not. I’m vegan, but I’m not naive enough to think that that’s an option for everyone. But the industry (and our consumption) can’t continue the way that it is.