Most racetracks in the US now ban the sale of registered horses for the purpose of slaughter and a racehorse owner found guilty of it can be banned from that track.
Since there is currently no consensus and no regulation created to determine what is "humane" for horses, and the captive bolt, which is not humane, was accepted as humane before, it is a guarantee that if slaughter were returned to the US without any regard to the use of captive bolt, the same technique can and will be used.
You are right that breeding is all about money, too. I agree that breeders share a responsibility. However, in the dog and cat world, that responsibility has not magically diminished the number of unwanted pets euthanized every year.
There are three ways to prevent things from happening in this country. One is to create and enforce laws against it, which has anywhere from negligible to moderate effect. The secondd is to make it financially untenable, which is much more effective.
The third way, and the one people most often ignore, is to create a financially lucrative private industry (which means zero to limited government regulation) that encourages the opposite of what we're trying to prevent. This method is proven to be most effective, historically, and in time, becomes the accepted norm.
One of the "old customs" that has fallen by the wayside, due to changes in how ranching has been done, is that ranchers used to run their horses loose on the range and every four years would go out and shoot the stud and allow one of the two year old colts to take his place. This was done to keep inbreeding to a minimum and because it was cheaper than gelding. This custom was turned over to the government as a method of handling the wild horses. The "mustanger" was born, who was given bounties by the government to "remove" mustangs. Well, I won't go into the whole story, but over the years, with pressure from private citizen groups, and changes in federal funding that added or removed money with each new administration that came into power, the current result has created a virtual impasse between those who wish to "save" the wild horses, and those who would rather have them removed.
The book, "Wild Horses and Sacred Cows" is an old book, but a very good read.