Yeah, IMHO Signal is the right balance of usability and privacy. Problem with not having a user ID is that you can't easily use the application on multiple devices at the same time and if you lose the device, or don't properly migrate to a new device, you will have to start over building your connections to others.

But the real issue with no user ID or centralized platform is discoverability. Same reason things like gpg for email never caught on. You can't just type in a person's phone number, username, or whatever and start talking to them. It only works if you have another line of communication with each person to set up the connection. This is usually the deal-beaker.

But the problem with user IDs is that anyone can create as many as they want and use them to avoid spam and abuse filtering. So that's why phone number is used by Signal as a unique identifier. It's not 100% unique, but it's good enough to deal with all but the most determined abusers.