The reason why I came up with it, is that we have quite a few threads on dc that aren't very popular due to their very special area of obscure geeky interest, which get buried in time.
The fact that the forum pushes older posts with less replies down into oblivion by default makes the entire thing feel kind of like digg ( which isn't a good thing :) ).
really old threads should be closed, anyway.
Only if they have become irrelevant or deprecated. If it's just information or a topic that nobody at the time had interest in or knowledge about, it's perfectly fine for it to be revived later on if someone does come along with that interest.
Imagine if scientific researchers could only work on an idea until it gets pushed down because of newer more popular posts, then having to constantly re-start their project. Nothing much would get done. That's how I feel forums work sometimes, the same stuff keeps popping up now and then, and gets discussed without depth, or mediocre-depth until it is forgotten. Except for the very popular topics (our huge note-taking thread comes to mind...)
Sure, you can search for things using the forum search, but that wouldn't help you find discussions you wouldn't think to search for.