Yeah, that's going to be the next big ecosystem challenge for WebAuthn in my opinion. Ideally platforms would support 3rd party password managers, so you could manage your keys without having to tie your credential database to any one specific platform.
I'm sure as the popularity of WebAuthn grows there'll be more and more demand for cross-platform compatibility until eventually the big players are forced to implement it, but it's going to be kind of rough in the short term until then.
For normal passwords they currently seem to support third party password managers just fine, including direct integration in Safari and the iOS keyboard.
You can also export everything from iCloud Keychain to use with another password manager.
They could always use Passkeys as an opportunity to lock things down but it would be in direct contrast to what they have been doing with password management recently.
That would be nice. I used Hanko for one service I built that uses WebAuthn+email, so new devices get a one-time code in their email and existing just use WebAuthn. It would be kind of nice not to have to rely on email, but it's better than making people remember passwords.
I'm sure as the popularity of WebAuthn grows there'll be more and more demand for cross-platform compatibility until eventually the big players are forced to implement it, but it's going to be kind of rough in the short term until then.