People like getting their answers upvoted and quick. Especially if a lot of effort was put into them. Some of the older questions are quite interesting. People might be getting a new perspective on an old question over time as well.
- re-asking an old question is not allowed;
- most up-voted / accepted answers are always on the top by default;
- upvoted comments are not visible on your profile;
- comment upvotes don't get added to your reputation;
- posting an answer to an old question will only notify the person who asked the question.
This way SE discourages helpful additional answers to old interesting questions. Instead it encourages posting a minimal answer as quick as possible and then extending it once people start upvoting.
Create a bounty for extra answers on an old question
Show a mix of newest and most-upvoted answers on the top
Show top upvoted comments on user's profile
Add comment upvotes to user's reputation
Send more notifications about a new answer