If you wanted to delete accounts, you could minimize the chances of this happening by only deleting those accounts which are "more dead" than others.
such as those with no activity AT ALL since joining, and set a minimum time requirement, say, five years? that would get rid of a few users.
it would have to be a long time, because lurkers could still be there.
can you see how often people come onto the site? or, when/if they log onto chat? that may also be helpful information to have, if you guys ever decide to have your own little purge.