It has been some time since we announced this and it is not going to be reconsidered, I'm afraid.
Simply because your users may not know their email address doesn't mean you shouldn't move into a more secure environment. It is kind of like stating that you lost your house key so you can't change the locks 🙂 . There have been many suggestions for getting your members updating their email address and getting used to this way of life now before it is fully removed in version 5. Making announcements and changes now that ease your members into this will improve the success come version 5. Enable display name and email address login now, set a date in the future that you will remove display name logins, and make plenty of announcements about it so your members are aware and know how to check their email address on file for their account.
Many who have gone through this process have expected to be flooded with contact requests but really only get a handful or less so getting in front of this is the best plan 🙂 .