There are quite experienced users who keep the automatically assigned username. For example, see here or here or here.
As you can see in the links above, there are many users with >1k rep who have kept their randomly assigned usernames. If that's what they want, why should we pester these people to change their names? This is not a social site, it's a Q&A site. Some of us like to engage in the gaming aspect and treat it like a social site but that is not its objective and we shouldn't try and force it.
In short, let people name themselves as they will.
