Users can have a different key pair for each website. Also, signatures may not be re-used and are only valid for a few seconds. Try to create a key pair and login to the test website.
Webauthn doesn't require separate keys per site and user vigilance to stay secure, because it has a-not-so-simple challenge-response protocol that is site-specific. For end users Webauthn is easier to use: just press a foolproof button.
I don't want to sound too negative. ed25519 keys are neat, and have fun implementing software using them. Let's just be realistic that a practical cryptographic system needs many more features, and Webauthn has them for a reason.
A few seconds is plenty of time for an automated system to cause harm.