This concern is valid depending on your threat model, but improvements are quickly coming to this area. Apple, Microsoft, and Google have announced that they will allow syncing credentials between devices in their ecosystem (fully end-to-end encrypted, like a password manager), and several password managers have started to add software implementation of FIDO authenticators to their offerings. Hopefully in the near future we'll see the big vendors open up their APIs so that password managers can use the device hardware as well.
Of course, multi-device passkeys will not work for high assurance situations, as TFA calls out, but that's not likely the case for any of the services you currently use a password manager for anyway.
As it stands today, using passkeys with a software implementation, barring implementation bugs, is no less safe from credential loss than passwords in a password manager.