So if a newborn baby cannot digest milk, it would probably die, unless other alternatives are available?

There are rare cases of mothers who have to give their sprogg formula because the baby doesn't want breast milk. Maybe the sugars in human milk are fundamentally different. Maybe human milk normally has other elements that promote the digestion. And even newborns can select foods that work for them, or not - eventually. That's evolution for you.

And

MJ from the Ceilidh band "the Committee Band" pointed out (as an expert on such stuff): allergy is potentially life threatening and affects the body (Immunologic? anaphylactic shock etc), as opposed to an intolerance which affects the gut and causes stomach disorders (and diarrhea etc) but would not, of itself, cause death.