It would only be in vain if the thread presents no new discussion. A lot of people don't like replying on older threads when the discussion is outdated, and because most substantial users here will read from the first page on wards to catch up with the discussion older threads can sometimes be problematic if that information is now outdated. For example, there's numerous threads over in the speculation section which are repeating topics from several years ago, but because of the ongoing nature of the value, and price of Bitcoin ongoing discussion is going to happen. Sure, we don't need several guides on how the trust system, because one guide is enough, but not everything is black, and white when repeating topics occur.