The thing I don't quite get about PH is we are discouraged from responding to old questions. And yet we can't ask the question a second time. So what if the old question yielded sucky answers, or maybe there is simply more to say about a specific question. What recourse does a person have? We can't revive the old question, but we can't ask it again, either.
Where does it say that we are discouraged from reviving old threads? It happens all the time. There's even a badge for it.
As Eric said, " Duplicate questions dilute the resource." I'd add they can fracture the resource if several conversations on the same topic are discussed separately.
The ideal process is to search using a few possible tags and find any existing questions that are very close or on exactly the same topic. If you decide to start another question, you can include a link to one or more existing ones and explain why you've chosen to branch off from them.
In my mind, this site is full of thoughtful, articulate conversations that deserve to be included in relevant questions.
It's easy enough to miss older questions if they happen to have different tags than you thought of, and if others bring them up in their responses that's a good thing as your thread is thereby enriched and the tags on all questions can be reviewed to make sure they share tags that are the same so they all come up in future searches.
I was surprised by your statement about discouraging older questions because many of the active questions on a typical day date back weeks or months.
I think this is also why people are allowed to retag questions. I had searched for an answer on something, found nothing. Then I asked my question, and it was promptly closed. Had the original question shown up in my search, there would have been no duplicate.
That said, i think it is good to go back and look at old threads. People forget about things not on the first page of questions, but we sometimes need to revive a conversation because of inadequate answers, or in the best case, better information.
I think there were two questions that got closed recently that are a perfect illustration of why I close. Both were from ex-veg*ns new to paleo. Question 1 was full of multiple loaded questions (implying the meat caused bowel cancer, questioning that paleolithic people actually ate very much meat, etc.) that had been answered before. It was like a ticking moderation timebomb. So I closed it. I closed it because it was loaded with multiple ideological questions that have already been discussed here. Question 2, was one woman's very specific question about trying to hack her transition to paleo. I left that open, despite the fact we've had similar, but less specific questions in the past.
Replying to an old question brings it back onto the active question list and often results in some conversation. I know that is not exactly what you are looking for. Duplicate questions dilute the resource. However if you have a question you may need to rephrase it to get answers closer to what you are looking for.