I don't understand why you say that last bit - to my mind the problem (and its solution) are quite simple:
- Yes, I also think it a very good idea to try to reduce the unanswered posts to a minimum, if only because people trying to answer them will have an easier time of it (i.e. not having to wade their way through hundreds of posts that aren't worth answering, for any of the reasons mentioned earlier).
Getting rid of the very old unanswered posts should be quite easy, too:
- reply, this alone will make the problem go away

But don't just reply with a "unanswered for a long time, so there" message, since that will accomplish nothing.
I propose answering those with a question to the OP on whether he still wants an answer, thus letting nature take its proper course:
- if he does, he will reply, and the post will show up in "last 24 hours".
(or re-post it if he's smart)
If not, he won't reply, and the post is off the unanswered list
I'm totally not worried about sage advice vanishing off the forum in this way, since the fact that the OP does not want an answer can mean one of two things:
1. He got an answer elsewhere, and so can anybody else; if he was feeling social he would have posted this info in response to his own thread and might even have tagged it SOLVED;
or
2. he doesn't care, so why should we ?
Votes, please?
I think I might start on this anyway - answering the insanely easy ones, but asking the Question for any that are left orphaned for a looong time.
My response would look something like this:
" Dear Gentoo user,
Your post to the Gentoo forums has not been answered in [1/3/6 months' time, a long time];
I am hereby asking you whether you want to pursue this matter further, or, if you have found an answer since then, whether you would care to close this topic, optionally posting the solution or information that you found.
If you still seek an answer to your questions, consider simply re-posting a new one.
Regards,
The adopt an unanswered post effort"