They already are to many blind.
Every bill is a different thickness and there are blind people that can tell the difference.
There used to be a blind man that ran a magazine etc, stand in the lobby of a 13 story building in L.A. that people tried to trick for change and I never heard of one time that he didn't know the size of the bill that was handed to him. He would rub it between his fingers and know what it was.
Hi, my name is Bill, I'll be your twenty. Talking currency, get it?