[palata]

Because you forget that you need to build your contact list (if you don't use the phone numbers), which for 99% of users (including me) is a major pain.

Then your backup idea works, but kind of sucks in terms of UX (at least in Signal's point of view), and therefore they decided to go with phone number first, and work on going towards usernames later.

Don't worry, you haven't invented anything. It's just that your idea is not at the level of UX provided by WhatsApp/Signal, and the solution is more complex than you think.