While nobody expected books to have variable font sizes, the fact that ebooks do allow it to be adjusted means that people with deteriorating vision may still read them.
Yeah... and once you're no longer paying by the page there's really little upside to using a small font you have to squint at or use margins that are too narrow to easily scan the page. I have no idea how long the books I read are, but I probably read them a few hundred words per screen simply because it's way easier to keep my place. Average for a small paperback exceeds 300.
In the early days of the Gutenberg press when most were illiterate, they would gather together and the one person could read would read to the rest of the group. So, arguably, it was both easier and more inclusive for a blind person to read what there was to read then than now. At least they didn't have to rely on any special accommodations.