Another concern: as someone working on a large web development team, I think that maintaining a separate “non-visual” site would entrench a two-tier approach to quality and completeness (noticing and fixing issues, adding new features, etc). If you treat them as two separate applications they will inevitably diverge over time, despite best intentions. You might even end up with slightly different URL structures on each site, making it impossible for a blind user to find the corresponding audio version of a certain URL they found on social media, for example.