In this post Jeff Atwood argues in favor of keeping a post that is off-topic not only un-deleted, but open. He says:
In my mind, there are three broad guidelines that determine whether a
question is appropriate for Stack Overflow:
- Does this question match the criteria provided in the Stack Overflow FAQ?
- Is this question accepted by the community, as reflected in upvotes, favorites, views, and answers?
- Does this question teach me anything that could make me better at my job? Can I learn something from it?
Then later says:
As Meat Loaf once said, two out of three ain’t bad.
I think that's a reasonable policy to keep in mind both when closing and when deleting questions.
Some people can believe in a die-hard policy that closing is a temporary state and questions must either improve and be opened or be deleted. I think that policy is arbitrary (closing isn't a temporary state due to a law of physics, it's temporary if we make it so, and the question is whether we should make it so), and that it makes more sense to say that questions that are out of scope, but popular and useful, but maybe not quite popular and useful enough to be open, should not be deleted.
One inevitable consequence of this is a degree of subjectivity, and therefore a certain degree of perceived (perhaps actual) unfairness. But a little subjectivity can be a good thing, and I think it's preferable to the alternative of removing useful and popular content from the internet.