Never really got this argument. We all also have names, which we “display” in the same contexts (conversation, email signatures, social bios, etc.). If you said “hey, my name is Tregetour, but I’d like it if you called me Treg,” and I responded “Nah, your name was already determined, it’s silly to believe anyone can enforce a preference about their nickname,” sure, you couldn’t enforce that I called you a nickname, but my response would still be a) rude and b) very weird. Societal niceties aren’t a matter of legal enforcement, there’s just a polite option and a bizarre option.