Could you? Yes. But there really is no point— biometrics alone are only a single factor for authentication.
You should have at least two of the three— something you are (fingerprint, facial, or retinal recognition), something you have (badge, token, secure device), and something you know (passphrase).
Authentication is, explicitly, the process of validating that you are who you say you are. Like biometrics, your username is part of your digital identity. So you are correct in arguing that biometrics alone is little stronger than a username, but by definition, both are part of authentication.
That said, to securely authenticate your identity, you need to use multiple factors.