When a user signs up , their username is something along the lines of "user########"
Granted, in the welcome email it does say something along the lines of "Head over to your user account page to chose a display name!", however only a few people actually read that email.
I'm not saying we should restrict people from changing their display name again in the future - Everyone would still be able to do so. It's just that when you sign up you choose an initial display name that will be what your first display name is instead of "user########".