I live very remote so hard for me to join any type of group, but over the years we have rescued more horses than you can imagine. We have several right now in fact. I do help out others by donating money for feed or helping them find homes for horses. Your experience may be different but I have found individuals that care do more actual horse rescue than many groups. Some groups are shady and their interest is not always the horse.
I am glad you are going to be involved in helping horses; many sign petitions, voice how terrible things are but never actually do anything to help the horses.
You are right that there is no real answer for bad owners, but wanted you to see the reality of the situation, large numbers of horses are unwanted just as large numbers of dogs and cats are unwanted. Many dogs and cats are euthanized because people breed them without reason or just too lazy to keep them from breeding- same with horses.
Rescue is great when possible, but there really are not homes for all the unwanted horses. Count your blessings each time you are able to save and find a good home for a horse.
I bet not more than I can imagine... but I’ve been involved in the issue for some time. I’m always glad and impressed by people and groups who are actually willing to help.
There actually are homes for most of the ones going to slaughter. As the market in recent years has shifted from pet food (they won’t buy horse any more) to human consumption, the kill buyers are looking for younger, healthy horses to slaughter. They are benefiting now, not from an actual glut of unusable horses, but a poor economy that has dropped the sales prices for even good sound horses down into the affordable range.