[slg]

>It would help surface questions that do need updating.

This is the biggest issue with out of date answers. No one is browsing old questions when they already know the answer. They are browsing because they have the same problem and they got there through search. The user can leave a comment in that question but that won't really bring any attention to the issue. This can lead to the question being asked again and then quickly closed because it is a duplicate of the question with the outdated answer. That serves to worsens mod/user relationships regarding duplicates that has already been brought up in this thread.