Accessibility: You will not always be able-bodied
Tech still tends to be a relatively young person's career, although I haven't figure out yet whether that's because it kills us faster or we get tired of it or we get rich and retire to the Caribbean. (I'm really hoping it's the third one.)
Having been a young person once and a middle-aged person now, I can confidently say that we humans do not know what it's like to lose a fingertip until we're looking at it on the kitchen counter. We can empathize, cringe, even get nauseous at the thought, but only those of us who have lost the tip of a finger (or toe, I'll spot you a toe) can nod and go "Yup, and here's what that experience is like."
On the other hand, all of us, experienced or no, can imagine and empathize with someone who's lost the tip of a finger. Giant bandage, itchy healing, difficulty typing, doors are a bit of a pain.... That's important, especially in light of Thing The First and our internalized stereotypes. When we take a group of people and say "I'm not one of them" or "I don't know what it's like to be one of them" it's easy to say "They're not important; I'm building for people like me."
But who are the people like you? When we say "They have an accessibility issue, in that they can only use a keyboard," do you picture someone who has a severe and permanent disability that prevents them from using a mouse, or do you picture a fingertip wrapped in gauze two inches thick?
The point of An Alphabet of Accessibility Issues is that anyone at any time for any number of reasons might find themselves in need of your accessible product. They might be permanently disabled, they might be temporarily disabled, they might just be distracted or have their hands full. They might be older, they might be younger, they might be exactly like you. Because we cannot predict who our users really are -- and for that matter we cannot predict our own health from one day to the next -- we have to build for everyone.