Given how few rides occur (less than what a single taxi driver sees in a month), it seems like the sort of thing Uber/Lyft could subsidize, particularly if all they commit to is the incredibly crappy service levels you get from the Taxi company.
The really evil part of me is thinking, and maybe I've got the wrong end of the stick here, like this:
When you request a ride with a taxi-service, it's up to the discretion of the service which car to send. You need something for a wheelchair? Fine, they send you what's available from their fleet.
When you request a ride from Uber or Lyft, those two are just middlemen who put you in touch with one of a thousand little companies. Potentially, each one is available.
So, here's the evil bit: require each ride-providing service (a taxi company or an Uber contractor or a Lyft contractor) to provide a minimum of one or two ADA-compliant vehicles.
The independent contractors for Uber and Lyft immediately go away, except for the one or two that are so wealthy as to be able to afford a ramp-van or other compliant vehicle.