#1. I think when people hear discussion about accessibility, there is an assumption that the only people who benefit are folks who are blind, deaf, or with severe physical impairments requiring a wheelchair.
In fact, web accessibility best-practices also aim to reduce barriers for folks who have e.g. ADHD, autism, dyscalculia, trauma disorders, poor network connections, cheap/low-grade devices, difficulties with mobility and motor control, stroke, color blindness, memory impairments, among many, many others -- ultimately lots, and lots, and lots of people. Also, web accessibility standards cover not only physical and intellectual/cognitive disabilities but also socio-economic and situational impairments (e.g. you're situationally one-handed because you're carrying a baby).
On top of that, it turns out that just as ramps and lifts are quite helpful for nondisabled (situationally or otherwise) folks in many circumstances, so too are websites that don't violate web and accessibility standards by hijacking browser functionality and behave predictably.
#2. The idea that folks have about how "those members of the population are a tiny portion and the business value is minor" is very strange and patently false: the vast majority of people on planet earth will experience disability at some point in their lives. One in ten Americans has a disability, that rises sharply with age to nearly 1 in 2 for older Americans. These aren't difficult stats to find sources for -- so I think this particular argument constitutes a certain level of willful ignorance underlying much of the difficulty disability advocates face.