Then they can just rename their account and come back. It's that simple, just be inactive for a while (1 - 2 years I guess), and your account is on, let's say "hold". If your name has not been taken, then you can instantly reclaim your account and become active again. If it has been taken, then just rename it. Is a username that important to a person? If they spent their hard earned time thinking of a unique name for the previous one, then they can do it again.
I mean, there has to be hundreds of accounts made for the purpose of reserving the name, but never using it.