If I share the same private key for an account with "n" devices, losing (incl. theft of) that key will lock me out.

If I have "n" private keys for an account, I can use another private key and revoke the lost/compromised one. It's that simple.

Your secure enclave is not much different electronically from a smart card with a biometric password actually. People think Passkeys as SSH keys on disk, but it's more of a long private key on a single-way secure enclave. This is why people cry "platform lock-in". It's platform lock-in, but it's a secondary effect. It's actually a "proper HSM, but integrated".