I’m sure most people got the username they wanted, definitely more than 1% anyways. But if more than one person had the same username, people would be “screwed out of their username”
Now if someone wants the same name they’d have to add numbers at the end, or something stupid like xX_
But they went in order of sign up, so if they had used this model from the beginning those users still wouldn’t have been able to get that username.