Horses did quite well as a species prior to man's figuring out that they would put up with a rider. Much of what is said to be necessary placates the owner more than it helps the horse. In some ways, horses are like cars. You have a car. Regular maintenance, gas and oil are about the only essentials.
Horses are kind of like that, too.
As long as you take care of a few basic things, everything will most likely be just fine. Just like your car, they may have the occasional breakdown, but the majority of them are repairable. Finally, you, as a horse owner, have to be reasonable. There are not reasons or answers for every problem. Working on horses is a challenging undertaking, at best.