The previous post sounds somewhat negative — although also realist — and really is about living memory: Some fine day, the last person who actually experienced World War I is gone; the last person born into slavery in the USA passes; and so on.
Yet, there is another side to this: how close we are to things in the past, and although the living memory horizon passes, we are still only a few degrees removed from the past. One story I always liked was this: a person who wrote for the local newspaper, and whom I was once in a room with, mentioned that when he came to Copenhagen to study, he rented a room with a very old lady. She once told him that when she was a child, her father came home one day and told her that Hans Christian Andersen had just died. See how few degrees suddenly take us back to a past that seemed so very distant?
So while we fade from immediate memory, there are things we leave behind that will persist and connections in degrees. We were actually here.