When you own a horse you should be dedicated and loyal to him until his last day. In time he will get old and that might cost you, because he`ll need a lot more care. Sometimes owners can`t pay those treatments and their only solution is to give up the ownership.
There are many things that you do for your horse instead of just giving him to some strangers. And if retirement homes are not an option, maybe you should create a local horse community network! You`ll see how practical this idea can be. It`ll be helpfull in many ways and even for rehoming a horse, so feel free to create this kind of network.