I want to manage passkeys myself. I don't want to use iCloud Keychain sync, and I don't want to use 1Password. This is a non-negotiable requirement for me. Other than that, I'm fine with the idea.
I want to see proper migration paths between iOS, Android, Linux, Windows, Mac, BSD etc - so that there is no platform lock-in or inertia that prevents you leaving one platform for another, and quickly (in under 30 seconds) moving everything across.
At a time when competition authorities are clamping down hard on platform dominance, self-preferencing and lock-in, passkeys risks becoming a pain point if there isn't proper, genuine, portability.
I'm also not keen on the lack of self-sovereignty of passkeys as you suggest - this feels like a "by the back door" way to force everyone into using "Sign in with X" type federated login. When your cloud account is locked out by Google AI, you want to be able to sign in to your other services! People have already started to discover the risks of Oauth type login when their bigtech company locks them out.
Signing into something with your ethereum wallet works well, you get to pick how you store your keys and sign (brain, paper, encrypted file, hardware wallet) and if you use something like argent you can even have social password recovery if you want it for low impact applications. It's so awesome and it just works. I wish more things used it.
Whilst it is blockchain-adjacent it does not use a blockchain for identity verification, it just checks the owner has access to a particular private key.