Replying to Do NS Usernames expire?

Question mainly for mods, but this was in no way important enough for a PM. Do usernames ever expire and go back into the pool?

For example: The name I've been best known for is "CaptainObvious." I had to add the period because w/o a period, it was already in use. However, if you look at the user "CaptainObvious" he or she hasn't logged in since 2002. Likewise, username "TheCaptain" hasn't logged in since 2003. Is there a reason why you don't let those accounts die after a decade or something?