Regarding SMS - it's not always an alternative to SMS since it uses data. I know data is more popular than SMS in many countries, but when you hit your monthly limit, you want to disable all data.

Regarding the simplicity of using a phone number and how it would be very easy for all users - Wickr provides basically the same service, is more popular, but uses usernames instead of phone numbers.

The fact that something is open source isn't always a clear win. OpenSSL, for example, is open source and yet they seem to have many bugs.

For the record, I use Signal almost exclusively for communication. I just wish it was more protective of users' privacy without having to put in a ton of effort.