Great idea. I have thought about something similar. One forum I go to has a six month wait... after that the account is deleted automatically. It would definitely cut down on the clutter and free up a boatload of user names.
Along the same lines, I wonder if there is a way to automatically lock a thread after so long, say six months? That would eliminate a lot of necroposting, and after six months the tech would have moved along enough to justify a new thread.