Every now and then this issue comes up amongst the gods and pmdev and we usually say that we will dispose of the "dead" accounts and then never quite get around to it. The only reason I personally would like to nuke those accounts is because usernames are unique and those 20k id's that are lost to the logged in once never posted crowd are thus unavailable for real users, and personally id prefer real users have priority for the cool accounts.

One of these days one of us will have to wipe out those accounts.