> designed by evolution over millions of years to be precisely what a mammal needs to thrive

Infant mammals. Adults consuming the milk of another species is recent. (About 10,000 years in humans.)

We have mixed evidence for the benefits of milk. It was a rich source of calories when those were scarce. Today, it might promote obesity. And good advice for some is bad advice for others. For example, milk boosts iron at the expense of vitamin D; that makes it favorable for light-skinned people at high latitudes [1].

Milk's branding as a superfood essential for kids is a product of post-war propaganda [2]. (TL; DR The U.S. government boosted dairy demand to feed soldiers. After the war, that production was threatened. So civilian demand was boosted.)

[1] https://pediatrics.aappublications.org/content/131/1/e144

[2] https://dairymarkets.org/pubPod/pubs/EB9505.pdf

  production was threatened. So civilian demand was boosted.

That makes no sense. When production is "threatened", artificially boosting demand would be the exactly wrong thing to do.

> When production is "threatened", artificially boosting demand would be the exactly wrong thing to do

There was wartime demand. Dairy farmers invested to meet it. War ends. Demand goes down. That capital must now be written off, re-purposed, or the demand re-started. We chose the last option.

Production as in productive capital or productive potential.

Some humans have the necessary genes to digest milk, some do not. Can't lump all mammals together.

> Milk contains a type of sugar called lactose, which is distinct from the sugars found in fruit and other sweet foods. When we are babies, our bodies make a special enzyme called lactase that allows us to digest the lactose in our mother’s milk. But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. As a result, if an adult drinks a lot of milk they may experience flatulence, painful cramps and even diarrhoea. (It’s worth noting that in other mammals, there aren’t any lactase-persistent adults – adult cows don’t have active lactase, and neither do cats or dogs, for example).

https://www.bbc.com/future/article/20190218-when-did-humans-...

> "Some humans have the necessary genes to digest milk, some do not. Can't lump all mammals together."

Note, adult mammals. Drinking milk as a baby is literally the definition of mammal. Even the platypus, which lays eggs and lacks teats, counts as mammal because they still produce milk to feed their children.

Then how do cats digest it?

Poorly.

Adult cats are generally lactose intolerant[0].

0: https://www.prestigeanimalhospital.com/services/cats/blog/wh...

Cats drinking bowls of milk is a TV thing or people who don't know better. Almost all mammals, including humans, lose the ability to effectively process lactose at some point in their life. Keeping the ability to process lactose as an adult is somewhat unique to humans and even then most will stop producing lactase before they die.

Is there a test to determine if you do make lactase?

Rapidly after drinking a glass of milk (or any of the many, many things that include lactose) it becomes very very clear that I am lactose intolerant to both myself and everyone around me.