If closing old threads is to save storage space then it is my belief that old threads should be deleted instead of simply closed. Just because a thread is "old" dosn't mean it's not relevant to a current problem. Old threads show up in Google results all the time, many of them still unsolved. Closing them prohibits updating possibly old, erroneous, or missing information, diminishing their value. If they were deleted instead of closed it would solve the storage space problem and possibly open a fresh new thread which can be updated. Of course the best solution would be to only close "Solved" threads and leave all others open.