@PublicLewdness As I have heard, the Signal protocol (or whatever is the correct term) is used also by other chat sw. It does not meen all those can interact with each other, but that Signal protocol is very good beyond its own platform. I agree the phone number is a drawback for Signal (is there any good pro with that, or simply the solution they have used).
@PublicLewdness Aha, ok, not an obvious relation to me. A major advantage though as end user with the phone numbers, is the possibility to easily find which of my contacts that have Signal, so I eg can send a message by Signal instead of SMS. Phone numbers are generally public open catalog info, at least here in my part of the world.
My reason for refusing to give my phone number is that it's tied to my name; address; and payment methods. No program is worth giving that up for when there are alternatives that don't need it. The most people would get if they got my email for other messaging apps is a burner email tied to nothing.