May I humbly suggest that after a certain given time, decided by you guys at admin, a thread should be viewing only.
Could be a year or something, and then it would give a parameter on whether it's OK to restart a thread subject as things have moved on.
As has been mentioned, it's an archive.
Of course, to be as reasonable as possible, as most of these are honest mistakes, then there are now grey areas. Perhaps a T&Cs decision on how long the life of a thread should have so as to avoid confusion for new members as it can be confusing when they are "dug up" so to speak. The recent (today) one in Legal/Visa etc was a good example, 2003 if I remember correctly.
Hopefully this suggestion is acceptable as my last attempt somewhat 'flunked it', to be honest.


Edit - You could even have an option for posters to request re-activation of a thread if they so wished to a Mod or Admin if they really desired it to be so.
Just thoughts/input.