Does it not depend on what the thread is about?
I do see your point but if the thread is about a buying a PC then 2 years later any advice given is mute because A the PCs people have commented on are probably no longer available as its been replaced and B say someone has a Vista problem and theres no solution and then someone discovers the fix and then reopens the thread to say what the solution is.
In the case of A then you're right theres no real point in adding to the thread but with B the person involved might have worked his/her way around the problem without providing the solution.
I assume you are not proposing someone from PCA going through all the threads manually and locking them but are proposing some kind of automatic lock?
There are a few other threads that have been on going and have not be solved and what about them? Do you think they should be locked too?
The ones I am thinking about is the MBS thread and the one about the Fujitsu Siemens laptops cracking up. Both threads occassionally get another post as people are still having the problems.