It is worth discussing the meaning or meaninglessness of something only if it concerns everyone. If some activity is strictly voluntary, then why these arguments about whether it makes sense or not?
You seem to keep forgetting that someone actually has to manually handle those reports, they are not automatically deleted. So its not only about those few who report old posts.
With that being said, maybe forum staff concluded that moderators energy should be more focused at reports that affect members (and therefore forum) more, which are of those recently written posts instead spending a lot of their time going throgh thousands of reports each month that affect forum members very little, if anything.
Now we see that no one else directs any energy anywhere. Nobody cares about these reports.
Hence small incentive like report badge or something along those lines. Don't get me wrong, I personally don't care about the badges at all but I am pretty certain that would make people report way more so why not. Only problem is that chances of that happening are slim to none.