As long as you have only one device in use to access all your accounts, your password manager won't be available as wel and your passwords in the password manager db anywhere on a private server or in the cloud will be of no use for you unless you get a new device and install your password manager on it.

It's literally the same with passkeys, as they are handled by platform password managers on all platforms. That is completely true for Apple, which will store your passkeys end-to-end-encrypted in the iCloud-Keychain. Once you have a new device, your passkeys will be there.

Caveats are (yet) with Google and Microsoft. Google stores copies of your passkey on Android in the Google cloud (IIRC end-to-end-encrypted as well), but not on Google Chrome on Windows or Linux. On macOS Google uses the iCloud keychain as Safari does. Windows passkeys aren't synced at all to anywhere yet. But the FIDO alliance has specified that passkeys should be synced so one can expect that Google and MS will complete their implementation in the near future.

Then there is an increasing number of independent, cross-platform password managers that have implemented passkeys, which are then saved just like the passwords are saved by those managers.

Bottom line: the risk of loosing passkeys is practically non-existant on Android and Apple platforms and independent password managers that support passkeys. Google and MS still have some work to do.

What's the benefit of using passkeys instead of passwords? Passkeys can't be phished or social engineered by attackers, and they can't be stolen from servers. Both happens to passwords on a regular basis.