It's of course fun to brainstorm ways to deal with that "problem". The one that just popped into my head was: add a way for the user to flag their account as inactive when they know they'll be absent for a prolonged period and don't want their account to disappear. Inactive accounts are exempt from the twice-a-year sweep which culls any accounts that haven't done anything since the last sweep and clears everyone's submissions.
To prevent abuse of the system: inactive accounts can't upload, post comments, etc.; setting an account inactive requires two weeks' notice during which time the account isn't actually flagged inactive, but is subject to the restrictions; and the culls are random - there's one in every six-month block, but they don't happen at regular six-month intervals (at most you know that there's always at least two months between culls).
