I think that Bontemps story demonstrates how it is the conditions of life for the old that represents the tragedy. This set of conditions is what causes the tragedy that unfolds. The "summer tragedy" is the decision that Jeff and Jennie must undertake. Yet, the conditions that compels them to do such a thing is where the tragic condition lies. The tragedy is that a couple that is rich with experience recognizes that their present and future would be fundamentally worse than their past. Bontemps' work examines the conditions that drive the couple to have to feel that what they are doing, "the summer tragedy," is the only path for them to pursue.
With that in mind, I think that they only undertake such a condition because of what their experiences are. The death of their children, the illnesses they battle, and the physical deterioration in a world that does not seem to care or offer help to cope with such a reality are all the tragic conditions that prompt them to see suicide as the only possible path. In this, the real tragedy lies.