@Hopeisallineed
Vast amounts are bred for these disciplines? Racing maybe, but not dressage/eventing/show jumping - many that are bred for it and don't make the grade go on to have homes competing at lower levels, or being pleasure horses for amatuer riders, but many are simply bred to be pleasure horses to start with - not many people breed with the express intention of producing a world beater, and even if they do, by the time the horse begins to show it's not going to make it, no one is going to shoot a perfectly good horse that can be sold to recoup some of the expenses or maybe even turn a bit of profit.
And the difference between racers and horses in those other disciplines is that they are trained completely differently, yes very few make top grade horses, but they are not just shot because they don't make the grade - apart from the obvious it would be a complete waste of the money invested in them and they all have the basics of a riding horse trained into them first, before the more specialised training can even begin.
It takes around 5 years from conception (almost a years pregnancy then at least 6 months with the dam) to knowing if a horse will 'make the grade' that's an expensive 5 years, with a lot of time effort and money invested. With the choice to sell on and make at least something back or shoot it at a dead loss, unsurprisingly most breeders/trainers choose to sell on, apart from the moral implications of putting down a perfectly good horse that's not a world beater, it makes no financial sense at all.
Racing, from my understanding, is different in the respect that the horses are trained to gallop from A to B as fast as possible, and without proper and extensive reschoooling, are largely unsuitable for the average rider straight out of training, and that's where the problem occurs.
I've seen first hand the mess that people who don't know what they're doing make of rehoming an ex racehorse, and it turns out far worse for the horse.
Increasingly owners and trainers in racing are taking more responsibility for the horses they produce to make the grade, by having reschoooling and rehoming programs on their yards.
Does more need to be done? Yes it does, because it will improve standards overall, but there really aren't hundreds of horses going to slaughter every day just because they don't win an Olympic medal.