People seem fine adding each other on Facebook without using a phone number. When I add people on LINE messenger I use their ID not their phone number. When I meet a new person and exchange some contact details, it is rarely a phone number. I would also like to talk to some people who I do not want to know my phone number. I think this tradeoff was a mistake for Signal.

Usability? Signal prevents backups on iOS and has no solution for someone changing a device (or even restoring a device from a backup) to carry over the conversations and retain chat history and group memberships. This is because it puts security above usability.

It’s also buggy in many other ways (e.g., sending safety number change messages when nothing has changed with the device or number; contacts sending messages and asking if it was received, etc.).

Signal is quite bad on usability compared to other apps.

You can just provide a choice, whether user wants to use a phone number and a real name, or just an anonymous login, not linked to anything. Why Signal doesn't want to do this? They don't want users to be anonymous, they want real names, addresses and GPS locations I assume.