I'm not comparing to some absolutely maximalist alternative. I'm asking how you get an equivalent product experience without the compromise (which would make everyone happy). I strongly believe the UX afforded by the compromise is how Signal has won all its users. The threat model and all it entails
is the value prop.
I genuinely believe there is a lot of commentary on this thread from people who have never designed a secure system. You never get 100% security and 100% privacy. Even if you only use public keys, web3 style, you're still a traceable public key--by definition not private. Okay everyone uses a fresh key for every action. Well now you have a problem figuring out who anybody is and whether you should trust them. Either trust isn't self-sovereign or it is. And we've learned time and time again that self-sovereign trust systems are akin to anarchy. Signal leverages the verifiable short identifiers available to a mobile phone, at the expense of 100% perfect anonymity when asking the question "has this phone number used signal". Literally everything beyond that point is 100% secure and as private as two public keys corresponding can be.
1. As a signal user, I don't want to see the threat model weakened so that we can include email anons, personally.
2. Even if we did, I don't understand how doing so in any way solves the privacy issue. How is email any more private than phone? If an email provider got phished people would be yelling the same thing "how could signal be so stupid to use email, don't you know it's insecure". Email providers can still be compelled into shenanigans, too.
3. Signal as a product has to facilitate a key exchange. I'm pretty sure you can checkout their source code and run their protocol and solve the key exchange portion differently if you so desire. You could have "signal without phone numbers or email" tomorrow if you wanted. As long as your users are willing to copy and paste public keys into their messenger, that is.
To sum up: the key exchange and distribution is the entire problem. And Signal presents an adequate solution: bind phone numbers to asymmetric crypto, add perfect forward secrecy and give people secure messaging. Surely it's not for everyone, but this incident in my eyes only further validated that this premise is solid.