> It's not optional because its purpose is incompatible with that.

I don't blame signal for forcing strong PINs, I don't know what's their purpose, but non phone number identifiers don't require cloud storage. The contact list (with the associated public keys) does.

What? How? You just need a key (password). If you are able to log into that "identified" then you are online with that and others can message you on that, and the network will route messages to your client.

There's nothing to store up to this point.

And if people want backups they can optionally enable that. Or the network could support multiple clients signed in for the same "identified" and allow those authenticated and authorized clients to sync/backup among themselves.