I wasn't sure where to put this, so please move if it needs to go somewhere else.

I understand the purpose of merging similar threads to limit the number of posts that are similar, but at what point do you not merge threads? Once a thread has 10-15 pages it begins to become very cumbersome and outdated, especially if the topic was began several years ago.

At what point do you kill the original thread and let a newer one begin?