You know, that is a good point. Far be it from me to encourage Apple to do more attestation -- to be clear, UX problems aside I don't want a centralized identity management service.
However, from Apple's perspective, this does kind of feel like the worst of both worlds. People have to update their devices to the most recent iOS version, apparently being signed in on an old device just turns off verification, apparently it's not even per-device?
So if that's the case, Apple has all of the downsides of attestation right now. Why also have the downsides for keys and in-band verification as well. It does seem like it would be simpler for them to try and have this be something that's tied into iCloud that gets set up only by the person who wants to be verified. Again, I'm not saying I want that, I don't want Apple arbitrating identities, but... why wouldn't they? Why have a system with both downsides?
I'm sure there are caveats I'm not thinking of, but it does seem like they could probably do this in a less federated/decentralized manner?