I don’t think it makes sense to call milk a good (or bad) food without additional context. If you don’t have a goal in mind for the food, you can’t say whether it would be good or bad at meeting that goal.
I think that milk was designed (by evolution) to be a good food for baby/infant cows. It might or might not be a good food source for an adult cow (ignoring the problems with efficiency loss for now). Baby and adult cows might want different ratios of protein or fat in their diet. Eating mainly milk might be optimal for a baby, while eating mainly grasses/legumes might be optimal for an adult.
(source: https://youtu.be/sXj76A9hI-o?t=1999)
I took this screenshot from a lecture on YouTube. I remember the presenter mentioned that carnivore milks have higher protein content because their babies grow more rapidly. And that human milk has more lactose because we have a larger brain relative to the size of our bodies.
I don’t think we should assume that food evolved under a certain set of conditions (growing rapidly, different brain to body size ratio, etc.) will still be “good” when those conditions are no longer present. It might be trying to succeed at a goal that is no longer relevant. Does that make sense?
I think you might have made a similar mistake in this article: Curiosity – Health “Experts” Betrayed America
algae has a lot of n3, and is eaten by fish, so fish are a good n3 source
Fish can accumulate pesticides or other chemicals in their bodies, in addition to n3. Our goal for eating n3 is to help our bodies and the presence of dangerous chemicals could make us fail at that goal. We shouldn’t consider fish only as a source of n3, but how it relates to our overall goals for nutrition.
We could form a goal like “obtain a source of n3 that is low in mercury” which might mean that certain types of fish are no longer a good source. Depending on our goal, the only solution might be to get the n3 directly from the algae, instead of using the fish as a “middleman”.
I hope the tone of this post isn’t combative. I think reading your speculation is very valuable. I also considered that you might be writing this post knowing that the audience is familiar with evolution, yes/no philosophy etc. and so you didn’t feel the need to go into detail. And that it wasn’t an error, just something you were aware of and had a good reason behind.