Hi Horatiu,
I remember putting this question to the SCN "caretakers" a few years back. And the reply I got from them is (what you already mentioned) about the volume of data. There is so much data that it is nearly impossible to go about "cleaning" the noise and leaving the "good stuff" untouched. A time-bound archiving is not a good idea either since it will relegate some of the old "gem" threads to the archives.
So when I became a Moderator of ABAP forums, I tried to "clean up" as much as possible. I began by wiping out those "Interview Questions" threads and threads those point to other link-farming threads. These threads hardly added any value to the forums. The link-farms and interview-farms formed a vicious whirlpool of useless noise.
In the ABAP forums, thanks to a lot of Active Moderators from time to time, most of the basic and oft-asked questions were dealt with efficiently. We maintained a zero-tolerance towards basic/easily searchable, copy-pasting, link-farming behaviour.
In the Career Center forum I collected important/useful/helpful blogs and made them as Sticky's so that the information is readily available and that members don't go about asking the same questions over and over again.
I also tried to set a few things right in the Wiki area trying to collate stuff and set things in order. But there's only so much one can do with the volume of data involved.
In conclusion, if every Moderator does his or her bit in the "clean-up" then yes, we can cut through most of the noise and get the useful information easily.
pk