Part of the pro-slaughter argument has to do with Thousands of horses wandering the streets starving to death, as one person put it. On the Pros and Cons section, this is answered with, amazingly, actual facts that dispel that theory. However, there still remains the question of what someone can do when they can truly no longer keep their beloved horse.
Hundreds of places will accept horses for donation, while leasing and private sales might also be an option, if the contracts are written correctly. Euthanasia with rendering is also an alternative that might need to be considered. You could also call a local rescue organization to see if they can help.
For an in depth look, with links and phone numbers, read about trusts.