How about something along the lines of this:
- When you think a question becomes stale (the top answers are no longer relevant, new information was created on this topic "out there" but isn't reflected in the answers), you can vote to mark the question as stale.
- When enough people voted a question as stale, it goes into "stale mode".
- "Stale" questions keep all their answers, but not directly visible - you have to click a button that says "show old answers". Those answers aren't counted when checking if a question is answered or not.
- Only new answers to the question are displayed by default.
- A question will remain in stale mode forever (or until reverted by mods/votes). Once a question gets enough fresh answers and enough time passed since it went into stale mode, it becomes possible to "restale" the question, moving all the "generation 2" answers into the stale mode hidden box.
Edit - alternatively we could mark specific answers as stale. Upside - This is a more surgical approach, no need to eliminate all the answers. Downside - if a question has a whole lot of stale answers, each one would have to be voted stale. Still, perhaps the upside > the downside.