Question Author
"What's the actual idea here, to get more people responding to old questions, or just to give them related threads where they can seek more information?"
The latter. Or, in cases where there are no answers, an alternative avenue.
"If the latter, there might not be much point in linking to old CB threads, for instance, or to anything very old."
See, old posts might be useful in law, especially when it's a rare question - I wouldn't want useful information excluded from users.
Excluding CB is a pretty good idea however.