My big issue with this argument is that usernames and passwords are a means to an end. That end is (in many cases) establishing identity. One can establish identity in other ways. For example: every morning when I open the door to my house after my early morning run, my wife greets me in the kitchen and asks me how my run was. She (thankfully) does not hit me on the head with a frying pan and call the cops about an intruder. My biometrics work awesome for establishing that I'm "me" in the real world. I don't think it's too big a stretch to imagine a world where my computer is as good at looking at me and identifying that I'm "me" as my wife.