The easiest way to good accessibility is to do it rather than trying to persuade people that theoretically can say no. It’s part of being a good designer and/or developer; to do otherwise is the undersell your skills and let down your employer / the client.

You can mention the liability potential from the ADA.

I've started to notice that customers are getting uneasy at the prospect of incoming legislation here in The Netherlands. I believe there's a new law in the works that makes accessibility mandatory in some cases, mostly pseudo-government related content though, so its scope remains limited.