I understand, philosophically, the idea that old, closed questions should be deleted. However, the issue that we have on our site is that there is a lot of controversy on a lot of closed questions. We don't have agreement right now on what should be closed and what shouldn't.
The system, in general, is biased towards closing questions. It takes at most 5 votes to close a question. (Often questions are closed with fewer than 5 votes.) While questions are acquiring close votes, there is no explicit way for users to vote to keep a question open. After the question is closed, 5 more users can vote to reopen a question, but this rarely happens for a number of reasons: the question can't get bumped to the top with new answers, the users who thought that the question should have remained open have already seen the question and interacted with it before it was closed, and they don't get notified when the question gets closed. And moderators are reluctant to reopen questions closed by other users unless a meta discussion about the question occurs. In addition, reopen votes expire fairly quickly, and there is a waiting period before a reopen vote can be reissued.
Despite the fact that there is no explicit way for users to vote to keep a question open while it is in the process of being closed, there are de facto methods for users to express their desire to keep a question open. When a user upvotes a question or posts an answer to a question, they are in effect expressing their opinion that the question is on-topic and should remain open. I understand that it is often said that "high votes don't equate to on-topic," but at the same time we can't ignore that high votes (or even just a few positive votes) indicate that there were users who thought the question had merit.
As a result of the bias toward closing questions, we have a lot of questions on our site that have been closed, and yet I (and others, I think) believe that many of these were closed in error and should be reopened.
We could discuss each of these questions on meta and try to come to consensus on which ones should be reopened, which ones can be reopened with edits, and which ones were closed legitimately. However, this activity will take a long time to complete, and it is debatable if this is a valuable use of all of our time and effort.
A viable alternative is to simply refrain from manually deleting closed questions en masse. By not deleting these questions, we honor the desire of the users and moderators who originally closed the question, and we also don't destroy the hard work of the users who spent their time and effort answering these questions that they believe are actually on-topic. This also allows the debate on what constitutes an open-worthy question to continue without putting an artificial timetable or deadline on it.
Stack Exchange has an auto delete feature (dubbed the Roomba) that automatically deletes questions under certain criteria. Closed questions with no positive score answers will eventually be deleted without any user action necessary. There really is no reason why users must go around deleting closed questions with any urgency.
To be fair, I think there are rare times when manual deletion of questions is warranted. We want to delete questions whose presence is actively harmful to our site or to our readers. Spam should be swiftly deleted, as should posts with abusive language. Sometimes we get questions where the OP decides to post their bank account number; those need to be deleted quickly as well. But off-topic questions with negative scores do not need to be manually deleted; the Roomba will clean that up. And for questions that are closed as off-topic yet have good answers from respected users, we need to recognize that there is not consensus whether this question is really on-topic or not. And waiting some arbitrary amount of time before deletion doesn't really address the issue. One thing moderators might not realize is that manual deletion by a moderator is final; it is not possible for ordinary users to vote to undelete, and it cannot be undone by any process except another moderator action.
I have seen people make the argument that you shouldn't look at answers in deciding whether or not a question should be closed; that you can't turn an off-topic question into an on-topic question by writing an on-topic answer. However, what that argument fails to take into account is that a good answer can show you how a question you originally thought was off-topic really is on-topic. This happens all the time; users read a question and immediately think "Too broad; This can't be answered concisely," or "Primarily opinion-based; This can't be answered objectively." They might issue their close vote, but if someone comes along and answers concisely or objectively, that answerer has proven the close-voter wrong. If an answer is written that is an on-topic answer, perhaps it means that the question simply needs a minor edit to be kept on-topic.
In conclusion, I don't see any need for our moderators to blindly follow some convention from other Stack Exchange sites in deleting closed questions without considering the opinions of the users who have voted up and answered these questions, indicating that they believe the questions should not have been closed.
Edit: I would just like to point out that my answer here is the compromise option. Some users want to delete lots of questions, not content with them being closed. Others (myself included) think that many of those should be reopened. The compromise solution is to simply leave them closed and not deleted, but able to be re-opened or re-closed if the consensus changes in the future on any individual questions, preserving without officially endorsing the work of answerers.