I guess use default admin username and unable to change it may create certain vulnerability
I tried a couple of panels, one of my fav is Vesta. Vesta doesn’t offer such a function. I guess this is the practice from Linux account, ie. the highest account, root, is not allowed to be changed.
From my personal point of view, I do and really long for such an improvement, because I am the only person in charge of my sites. I need, but not that urgently, a personalized username, if not to create a new one.
Must add an option for delete admin username and create another admin user