horses are indeed fragile, thorougbreds inherently so and becoming more fragile and unhealthy due to inbreeding and the vast over production of foals every year in the hope of breeding a winner.
They may live in a guilded cage and have fancy names, but their lives arent great. Kept in most of the time, turning out would be too risky if they got injured and couldnt run, too much money at stake. As herbvivores designed for constant grazing of low calorific food, being fed food concentrates leads to ulcers (present in the vast majority of racehorses), joint problems from intensive training at too young an age, bleeding into the lungs, and heart failure (not heart attacks as such), which can occur in any horse due to disease, but in racehorses is generally due to the heart giving out as the chest cavity is too small to accomodate the expansion of the heart during intensive excercise.
Plenty of horses will refuse to run and jump. We just dont usually see them on racecourses. They will have been dispatched long before that, either put down (the lucky ones) or sold on to be tranported to foreign abbatoirs or to foreign tracks or into a spiral of neglect in unsuitable homes.
Some might go to the legendary beautiful retirement homes spouted about on here. The wastage in the industry is such that there would need to be a race horse retirement venue in every town, but I dont seem to have seen them.
The argument about whether we can justify "using" or keeping horses is a few thousand years too late. They are domesticated. Like it or not, they dont have free will, they are subject to the whim of humans. That means that we have a duty of care towards them, and that includes not knowingly putting them into dangerous situations.
Jump racing is dangerous, therefore participants ARE knowingly putting the animal into a dangerous situation.