I like the one previously proposed idea where users that have been inactive for 5 years with literally no activity/ have a default place/ not verified be given a notification that tells them at in a few months they’re forcing a change on their username unless they click a “I’m active” box or something in the notification.
And if they’re not active, they’re given a new username.
There’s a problem of how they would be able to log on or something, with the new username and all, but perhaps that could be worked around. Like if they sign in with that old password + username, they get prompted/ notified of what happened, and asks them to use their new account name instead.
(And give them the ability to change their username for free free of charge as compensation.