And maybe those identities will be magically synced to and from central servers, which would not be so great.

The option to "sign in with" a third-party provider is already extremely widespread and most people prefer it.

But U2F/Fido identities are better in that the people who prefer to ensure their keys are in escrow with a corporate overlord can choose to do that, while security-conscious people can go a step further and use physical security keys or third-party solutions with different sync mechanisms.

Security is a nightmare when you trust systems that you shouldn't trust. That's for sure.

Going from mobile phone numbers to smartphones is not an improvement. It's just going more expensive and more monopolized.

The only improvement is that you don't have to link your account to government photo ID (which is mandatory in many countries to buy a SIM card).

Well, until we learn that there is a backdoor in their implementation, allowing any big actor to swoop in and either crack the crypto easily, have access to an unencrypted copy, or steal the passphrase.

Then we just have put all the things in the same basket for the convenience of mass spying, like we did with https, which any entity big enough can MITM by using their root certificates.