[judge2020]

If you did switch ecosystems, you'd probably need to do the whole re-enroll dance on every website that had your previous key. I don't see any sort of passkey backup/import functionality coming anytime soon simply because people don't change ecosystems that often, and those that do can simply keep both their iCloud and Android-based passkeys on services.

[Mindwipe]

I would wager the majority of iPhone users use Windows on their main machines.

[judge2020]

Passkey works on non-Apple devices by showing a QR code on that device, having the iPhone scan it, then a BLE connection being established to allow the phone to remotely sign webauthn requests. I only mention ‘switching ecosystems’ in the sense of moving between Apple and Android mobile devices.