- IMHO it is totally unfeasible to expect anyone having hundreds or thousands of answers to manually browse through past answers to periodically detect obsolete ones
- editing an answer long after it has been upvoted / accepted may introduce a risk of confusion and uncertainty, as it becomes blurry what the upvotes / acceptance were actually awarded for
- also, old answers may still continue to be useful, even if they are not the state of the art anymore, for those who for some reason need to work with old technology
- relying on new users finding and downvoting old obsolete answers is IMHO very inefficient with SO having well over a million posts
- IMHO it is a bit unfair to downvote an old answer which was good and proper at its time of creation, just because the public opinion about the state of the art has shifted
- merging fresh duplicates with their old original may easily hide the fact that highly upvoted / accepted answers to the old question have become obsolete in the meantime - even if a new answer is better / more up to date, it gets behind the older ones if it has less upvotes
So my suggestion is for cases when it is visible from comments to the old and/or the new question that the new question obsoletes the old one, to introduce a special two-way link between the two instead of merging them: "this question obsoletes this earlier question / this question is obsoleted by this newer question". Possibly together with locking the old question to stop further edits / votes, as there is no point modifying an obsolete post.
IMHO this would solve the above issues:
- no need for anyone to manually update or delete thousands of his/her old answers - we can instead rely on the whole community to determine the state of the art
- old answers are still kept for reference, but clearly marked as "archive"
- new and fresher answers are not shadowed by obsolete but more popular ones
Please feel free to post your comments or thoughts on this. This is still a half baked idea; let's try to improve it - and SO - together :-)