Seems like the @username identifier seems to be pretty well accepted, and most people would probably prefer that rather than a phone number. Similar to how the mass user base only cares about a domain name rather than an IP address.
You personally my feel that the phone number makes a great user ID, but I'd venture you're old enough to remember knowing people's phone numbers. I am too, but I couldn't tell you any of the phone numbers that I interact with on a daily basis today. In a few more generations, phone numbers may be even less understood by the younger users.
The thing about phone numbers, though, is that you already have a lot of them in your contacts. Centralized apps can leverage that.
Usernames are nice, but you probably don't know whether your dentist uses Discord, or what is their username if they do. With Signal, however, you would probably notice (or be informed) when they sign up.
You personally my feel that the phone number makes a great user ID, but I'd venture you're old enough to remember knowing people's phone numbers. I am too, but I couldn't tell you any of the phone numbers that I interact with on a daily basis today. In a few more generations, phone numbers may be even less understood by the younger users.