I see it from both sides.
The information in old threads is still sometimes valid and should be viewable. On the other hand you do see people replying to threads that have had no activity in years, which does seem a bit futile. Invariably the person needing the information is either long gone or got his answer years ago.
Possibly if long dead threads were viewable but locked for reply, which I think is what fhhuber was intending.
For instance: http://www.wattflyer.com/forums/showthread.php?t=58234