> A PIN grants you access to the service
Incorrect. The PIN does not grant access to the service.
If all you have is the PIN, you don't get access to the service. Therefore, its not the PIN that grants the access.
If you know my keepass database passphrase, but don't have the actual database file, do you have access to the services contained within?
And as acdha mentioned, the entire login workflow is radically different with security keys / passkeys. Its a radically different implementation of authentication with different guarantees.
Do you leave SSH open on port 22 with only password authentication? It's just the same as using SSH keys, just a difference in implementation.