Not sure if relevant to open source bug tracking, but I've worked in more than one company whose "old bug" policy boiled down to: If after we found this bug, we chose to ship an update that did not include a fix, then the bug is not important enough to ever fix and should be closed as "Won't Fix". In other words, customers have lived with this bug for at least two release cycles and the world didn't come to an end, so don't prioritize it. I don't agree with this policy but I think it's a pretty common mentality in the commercial software world.

EDIT: Additionally, I've never worked anywhere where the capacity/willingness to fix bugs exceeded the incoming rate. So net bug count minus fixes would always grow forever unless you regularly purged unfixed bugs. This seems almost universally true in software.