Apple's method of 2FA, where they display the code on one of your Apple devices for secondary confirmation, would be somewhat akin to the scenario of how WebAuthn could work as 2FA by asking for confirmation on supported devices. I find this quite annoying as I don't use an iPhone (a mobile phone is one of the few devices that you always keep with you).

That said, the key thing to consider here is security vs ease of use. The more you opt for ease of use (like writing down your password or saving your password in your browser or WebAuthn passkey or using a password manager), the more you sacrifice security with varying degree (as the risk of password leaking increases). I'd say the proposed passkeys is definitely one of the worse option here (akin to saving your password in your browser) as anyone with access to your device will be able to access all your accounts without even needing to know your credentials.