With so many people visiting SO for exactly these kinds of valuable answers, I believe SO should embrace them instead of the ongoing battle of rejecting them.

If cleaning up irrelevant questions can't be automated, how about having a "this is no longer relevant" button appear a few months after the post has been answered? Or having a [subjective] tag that has an expiration date on being indexed on Google.

Now that the bad ideas are out of the way, let's hear some others.