Linkrot: what to do?

In our older threads (“older” on the internet means about 3 months), the links to external material start to break. Especially important material is sometimes imported to our server. This republication raises copyright issues. Graphical snippets may fall under fair use provisions. For the complete articles in the NEW section, we have had good success in obtaining copyright permissions, although in most of those cases we have had some previous relationship with the copyright holder (and even then it has taken several letters to settle on the terms of the permission). Only once has external posted material been challenged (Boeing claimed their Columbia slide was proprietary information). But do we go through the many many links in our threads to get permissions so the material can be posted directly on our server? There are now about 500 surviving threads in the 2 years of this board, with perhaps 500 links wandering around. Or do broken links lead to discarding the thread?

Are there some scholars of linkrot and its cures that can give some advice? Examples of practices at similar sites would be helpful. Some other boards seem to operate on the idea of “don’t ask, don’t tell,” or “don’t ask permission but beg forgiveness.” But are there better ways?