Not trying to be the devil's advocate here (I don't support 3-day eventing), but the article says Jet Set had an "irreparable ligament rupture in the lower right limb". Leg injuries in horse are notoriously hard to treat, for multiple reasons. A horse weighs 1,000 pounds or more, doesn't understand that it needs to stand still to heal, and has to stand and move around to support its circulatory system. This is why broken legs and other severe leg injuries are still "fatal" most of the time, even now with shock wave therapy, IRAP, and other cutting-edge treatments.
Again, not trying to argue, just wanted to add some context.