I didn't want to put this in the main text...the two things I can think of that might be precious (aside from layer-9 considerations) are disk space and namespace. I know there are things that need to be done to reclaim namespace, and my understanding is that they haven't been done in the past several years and there's no sign that they'll be done soon. This just leaves disk space; you could put a quota freeze on "old" accounts or something. Disk is also something you can exchange money for, though I can understand a reluctance to do so for the benefit of people who aren't students any more.