The reality is that most of Signal's popularity stems from them using phone numbers for initial authentication. Obviously there are people who can work without phone number, but the general population cannot. Or perhaps more correctly, will not. They'll just continue using a system that does.
>Obviously there are people who can work without phone number, but the general population cannot.
I think you're wrong in multiple ways. First, you are asserting a false dichotomy. Using phone numbers as one option for authentication in absolutely no way requires using them exclusively. To take a fairly large scale example, Apple's iMessage does not require any phone number. People can transparently use it with their phone numbers easily, but it will also work fine and always has with any email-based Apple ID. No telephone is required at all.
Second and more foundationally, we can objectively observe given the popularity of a range of online accounts using email or just user names (Facebook, Google, etc) that the general population is perfectly capable of working without a phone number. The UX might require more effort, and onboarding in some countries might be easier of course with phone numbers.
The reality is that most of Signal's popularity stems from them using phone numbers for initial authentication. Obviously there are people who can work without phone number, but the general population cannot. Or perhaps more correctly, will not. They'll just continue using a system that does.