Passkey is some sort of specific unique key to a device allowing to use a pin on a device instead of the password. But which won’t work on another device.
Now I don’t know if that key can be stolen or not, or if it’s really more secure or not, as people have really unsecure pins.
As you point out, the single point of failure is access to the passkey repository. Of course, this will usually be 2FA, so much more secure than simple passwords which people usually employ.
One major issue, IMHO, is vendor lock-in. I’ve no doubt Apple is going to make migration away from iCloud a huge pain in the ass. It’s just another way they’re going to make it difficult to leave their ecosystem.
I’m also worried about backups. People lose access to their Google and Apple accounts routinely for any and no reason at all. Will these keys be stored in the cloud? If so, access to EVERYTHING is just a capricious random algorithm away from being lost.
I wouldn’t touch any passkey system which doesn’t provide a seamless way to migrate away especially if I’ve lost access to my Apple/Google account.
Having a seamless way to migrate away is itself a security risk, since that method could be used by attackers to compromise the key store. The migration path for any of the major players (Apple, Google, Microsoft, Yubikey) involves logging into each site you used a passkey with, adding a new one from your new passkey store, then revoking the old passkey.
Password managers that store Passkeys may handle this differently, though, and are your best bet if you want migration flexibility.
The method you describe is untenable for 99.9% of the population. If that is truly the only way to migrate, then this move to passkeys is a catastrophe for security. In the coming years, millions of people are going to be permanently locked out of important accounts. Accounts will be written about the clearly flawed implementation of passkeys by Apple and Google, and a whole generation of people are going to shun passkeys forever. Myself included. This is a nightmare for vendor lock-in. I can see why Apple and Google are so ready to implement this.