Let's hope your iphone doesn't get stolen or smashed and your beloved passkeys lost
Plus, none of the sites that I use passkeys for would I be locked out. They all have account recovery procedures just like if I forgot my password. And, finally, for the important ones like my Google account (where my email lives) I have other non-Apple devices like a Yubikey setup as additional Passkey/FIDO authenticators
How do you log in without having the passkey?
In theory if I locked down my Apple account with the advanced protection features that are optional, I could lock myself out, but it would be on me to save recovery codes/secondary passkeys/etc during enrollment in those optional configurations
Otherwise, I'd walk into the Apple Store and they'd be able to get me back in my account if I didn't have some alternate method to get in
Recovery key: https://support.apple.com/en-us/109345