To be honest I never liked the idea of having the threads auto delete. The main reason I never liked the idea is that you lose access to the discussion on the thread itself which I sometimes like to go back to.
It is a good point you made, I too have taken time to read these old threads. But I have noticed a problem where these threads lack info as to what light they are talking about (no pictures or details) and clicking the ebay link hoping to see what it was only to get that ebay page with that "we looked everywhere" blerb lol.
I agree that this would require a lot of coding if not an update all together to implement and id certainly be appreciative of it and I'm certainly not in a rush to see this come to be but it is something i hope for in the future. After all its the years of innovating and patching that has made this site what it is today.