Claiming that the solution is simple is the problem. Signal actually has been trying to add usernames for years. It involves an account, contacts book, trusting Signal, trusting Intel, SGX remote attestation, Raft, and passpins. They are getting reamed for it because it's not really possible to treat a 4 digit pin as a strong password but they're trying to do it anyway. It inverts the whole value prop of Signal on its head. That's my point here. The reason I'm saying use something else is because it's not simple like you claim. And it changes Signal's model enough that people are leaving Signal because it's not what they signed up for.

Put simply, telling Signal to add usernames is like telling the existing users to "use something else" because that's what Signal must turn itself into to satisfy the "I need usernames right now" crowd.