They do not at this time intend to drop the phone number requirement, as they see it as an anti-spam measure. Meredith Whittaker has said as much. Phone number privacy is a project independent of requiring phone number at signup, and it just prevents other users from seeing your number.
Usernames have not been released yet. When they are released, phone number will still be required for registration, but you will be able to hide it from other people on Signal.
What about full passwordless login?