There are several personal and professional issues that a user will encounter due to the lack of displayName being accepted AGOL or Portal.
i. A user’s email, business card, and other organization systems, etc. use their preferred name – not given name
ii. A user, foreign students/faculty/employee, may use an English-name rather than their ethnic givenname; and users who no longer identify with their gender will prefer to use their preferred name – not given name;
iii. When one is sharing maps, apps, and data within their professional community (or the public) and peers, it’s respectful for the organization's systems to use their preferred name by which their colleagues and professional counterparts use.
It is possible for a user to edit their profile and change their name but it is a point of frustration to have make such an edit when other vendor systems in their org commonly accept displayName - though, if syncing/update is enabled, any edits the user make would be overridden each time they sign-in.
A person's first name (and potentially last names with marriage or divorce) and how they wish to be referred to is a large part of a person's identity - having a given name exposed to their peers, colleagues, and public can be an uncomfortable or embarrassing experience.