Deary me. I have said this previously and will say it again, the reason human's hesitate to put an animal to sleep, and ultimately out of its misery, is not because we feel bad for the animal (the animal has NO idea, it will just exist one moment, and be gone the next. A horse does not have goals and plans for the future, it is not going to think of dying as wasted life), but because we don't want to go through the emotional pain ourselves.
Now, we can say what we like, that we feel sorry for the animal, that we are trying to save its life etc etc. but ultimately, it comes down to our own selfish motives.
As has been said multiple times in above posts, why spend thousands putting an animal through pain and discomfort, when it will NEVER be able to function normally, when you could have saved multiple other horses and allowed them to live out long and useful lives?
If you are going to 'fix' legs like this foals, treatment needed to be commenced from a few days after birth, not months later. The best chance of success is when the soft tissue and bones are still extremely maleable as a newborn. It has been left far too late for this youngster and to continue with treatment is a fruitless and self centred effort. I see these cases as people trying to play the hero and want to boost themselves up by 'saving' a life, at any cost.