Not a we-must-change-the-software problem but I am having this problem at the moment and found this thread looking for a solution. I can give an example of when you can have a good bunch of these cases, many more than mentioned above:
I recently imported 8,000+ topics from another platform into a fresh Discourse instance. A number of moderators are working hard to get things organized which includes doing a lot of edits (partly due to the old platform not having a similar concept of topic titles). So when users log in to our brand new site, sometimes almost of half of what they see right away is very old topics with no recent posts (especially if a moderator has just done a clean-up session).
Now, our users know that we are currently getting started on a new platform so they are not freaked out, some have noted that revisiting old threads has been interesting etc. But still, I would much prefer to have people focus on new topics to get things going after the migration.