why is this a problem? Surely there is 'natural selection' by the number of members continuing to post or not? If a subject is interesting to many and runs for weeks, why is that bad?
Well, not exactly. Even though one thread might fade away, there will be more new threads to take its place.
In the past (long before PDA or any of the newer guidelines), I thought that they best way is to let the community sort it out. If something isn't interesting to enough people, then it would "naturally" disappear.
But after taking a closer look, I realized that (for the lack of a better way to say it) the more "think skinned" members were overrunning the board and driving away others, without even knowing they were doing it.
I would really love to try an experiment and remove all guidelines from GCC aside from politics and religion restrictions. But I'm pretty sure we would end up in the same situation we were in before.
And even if we only direct people to the PDA when their threads become too graphic or turn into pure venting, people would still complain that posts are being arbitrarily locked.