I definitely understand how it can be frustrating at times, but other times it is nice to have all the information in one place rather than across numerous threads.
For example, if someone is asking who has experience with a particular product or technique, I think it's okay to continue to add to an old thread. On the other hand, it doesn't make a whole lot of sense to respond to someone asking for opinions about color or pattern on their quilt if the thread is 3 years old. Certainly they have already made that decision quite some time ago. I try now to pay attention to the original date of the thread so that just because one person made the mistake of responding to an old thread, I don't continue to add to it. As far as I know (I used to be a web manager for a small non-profit), there isn't a way to automatically lock all threads past a certain date. They can either be locked individually or they can be mass deleted. I would hate to lose all that valuable information & I know how much work it is for moderators to go in and individually lock threads on a large-scale basis.