U2F is (as its full name "Universal Second Factor" would suggest) specifically only a second factor, it doesn't make sense as your first or only factor.
WebAuthn can replace the entire authentication, because it can perform multi-factor authentication locally and then send a claim to have done so, optionally backed by attestation from a vendor saying they promise the multi-factor authentication is done by their product. For example an iPhone can have one press sign-in to web sites or apps using this technology.