I will be glad to see the random numbers go, they make me feel like I am a kid who has not yet learnt that "Susie" is already taken and is settling for "Susie74514" like 74513 Susies before her.
Sometimes you want to just be Susie around your friends, rather than having to think of a clever/unique variant.
And I'm saying that as someone with a very early account who actually stands a very good chance under this system of just grabbing my moderately common first name as the screen name.
You can do that with the proposed system, because your display name can be anything you want and doesn't have to be unique.
And for interactions you’d be susie#whatever
Me, I'm gonna treat it like a firstnick.lastnick format when it arrives at my account. I figure "crb3.cdp1802" is very unlikely to be taken before I get there, but, if I'm wrong, there're lots of other already-memorized device-numbers I can append.