In this day and age, the role of the royal family is little more than pomp and circumstance. There is no real government role for the royal family, so the figure head at the top of the family is irrelevant. Queen Elizabeth is a beloved icon and should not abdicate.
Queen Elizabeth is, by law, the reigning monarch. As long as she continues to have a sound mind and fairly good health, she should stay in that position of authority. She is fully capable of holding the position and she deserves to stay on as long as she wants to. If that means until her death, so be it. It is good for the country to have a sense of tradition and strength, which she gives with her longstanding, dignified reign.