IMO one of the biggest issues with the Passkey spec is that it doesn't provide a way to automatically rotate credentials. The entire security model relies on Apple/Google/[insert name of nonprofit they end up allowing through the DRM gates to avoid antitrust suits] being completely infallible, forever.

That's why normally private keys (like when used with ssh for example) are paired with something like a passphrase that should offer an additional layer of protection. But still, you (the owner of the private key) can access it. You should keep both your key and your passphrase secret. Not sure what passkeys are doing about it, but I still don't see any valid reason for the owner not to have full access to the key.

If someone has sufficient access to your computer (like being able to keylog and stuff) - it's somewhat late to worry about keys being compromised.