[lmm]

If you don't need an insecure thing, why bother with a username at all? Just have a secure password with appropriate requirements.

I've done similar things and it makes life hard. People need a way to refer to specific accounts, to talk about them, to troubleshoot. And for most sites your username is a public identifier, a way for other people to talk about you.

But look at the way AWS credentials work. They're generated for you by the system, and no-one refers to someone else's login. But they still, very deliberately, split your AWS key into a public part and a secret part. Because it's very useful to have a public identity for a credential.