You made a good point, esp. if your passkey vault is comprosed, e.g. Apple iCloud's credentials are leaked. signCount, incremented or not, would not help here in informing you of your hacked iCloud account – that would be dependent on iCloud's service itself for detecting and informing you of your compromised account.

I would still like to see big tech passkey providers implement signCount for the following 2 reasons:

1. It helps to push relying parties to implement signCount verification. Right now most relying parties do not implement it as many providers are returning `0` for `signCount`.

2. This would be an odd one, it helps against detecting leaked private keys of passkeys, if a malicious attacker, internal or external, manages to obtain the private key.