Usernameless always seemed like an optimization too far to me.
I think it's totally reasonable, and probably a good thing for users having to use their username at login. Especially as it reminds them what username they are using for that service.
I could totally see a situation where a user uses a Usernameless passkey for years to access a service and for some reason loses access to the Usernameless passkey, and then has also forgotten the username for the service, so cannot even start an account recovery process.
I think it's totally reasonable, and probably a good thing for users having to use their username at login. Especially as it reminds them what username they are using for that service.
I could totally see a situation where a user uses a Usernameless passkey for years to access a service and for some reason loses access to the Usernameless passkey, and then has also forgotten the username for the service, so cannot even start an account recovery process.