Let me switch this around and ask why have user names at all? Most sites these days allow you to log in with your email address as your "username". At that poi

Let me switch this around and ask why have user names at all? Most sites these days allow you to log in with your email address as your "username". At that point you should be able to choose whatever you want as your display name. The only problem with getting rid of the username completely is that the username is by definition a unique string, while display names or real names can be the same, so you have an issue with deduplication of display names. You can easily end up with a system like YouTube comments where people troll by using the same name and avatar as others to impersonate them.