The actual studies show there is a J-shaped curve relationship between endurance sports training and long term health. Most people in affluent societies are way over on the left side of the curve and could improve long term functioning through more exercise. The risks for chronic problems like heart scarring, arterial calcification, and musculoskeletal injuries only start to increase once you get over about 20 hours per week.

Elderly people frequently die because they get weak, then they fall and end up bedridden. At that point they don't last very long.