Accessibility is not all about making things accessible to the blind. It's more than that. To name a few: Making articles easy to read or to use your website via keyboard without mouse interaction. But also providing captions for the hearing impaired in videos or below audio files.
By turning off CSS you forget that blind people are different. Some can distinct between light or dark, others can see structures and the other group of blinds can see a few metres. The structure seeing blinds may use the structure of a page to navigate even more fast. And imaging you turn off CSS for all these different blind people? The result: The structure seeing blinds may get stuck with this new situation.
There are more disabilities like cognitive disease, motor- or hearing impairment. By turning off CSS you don't make your website accessible to these kind of people. For example: The users with cognitive disease need an easy to use website and easy to read text. [Hem