A large percentage of users benefit from good accessibility features. These don't just include screen readers, but things like being able to change colors and font sizes (manually, or to pre-sets designed to help people with deteriorating vision or colorblindness, et c.)

Most people's vision and hearing go to hell at some point. Practically all older people can benefit from a11y features—whether they know they're there, and know how to enable them, is another matter.