You're talking about real privacy, the critiques above are all about exposure reduction (incorrectly framed as privacy). Good retention policies are still important for situations like trying to delete something that you regret posting.

An example I could think of from the other site is the very common occurrence of posting some relationship questions and then deleting them later so that the person they're about can't stumble onto them. In that case you want finding the thing you deleted to be nontrivial enough that it can't accidentally be found. Someone with both the skills and knowledge about what they're looking for may still find it, because it was once public, but that's a different threat.