If I recall, BCO is fairly adamant that nothing gets deleted. Unlike MS or other forums, ARRSE comes in some ways closer to a historical archive than other forums.
I would also be strongly against any locking of threads other than those identified as pure oxygen thievery. We have a strong community with a common bond which many other sites don't have, and as people join the site, old threads may be of relevance to them and they may have something to contribute.
Even at that, the fact a thread from 2009 about snow is showing on latest threads shows the timeless value of the site.
Bare in mind as well that if you were to decide a thread had run its course and lock it or delete it, someone would just start another when the need arose, duplicating content almost.
I think ARRSE stands head and shoulders above the rest and the threads should be retained and accessible with the ability to add to.
ETA: See? This thread had ran it's course and is now alive and kicking. The irony.
What happened to Sluggy's dog journal thread then?