That would not be my choice when it comes to aged horses, but I suppose it may be more humane than sending them down the river to Lord knows what fate. I went to a local tack sale once, there was a man there complaining about the slaughter houses being shut down. I do not have an issue with that, .....what I did have an issue with......his mentality was " well I have a mare, we have had her since she was 12, she was a good horse, she's old now, but now what am I suppose to do? Without a slaughter house, I either have to keep feeding her and she isn't good for anything now, or I have to pay a vet to put her down". My thought is, if an animal has served you well, you have an obligation to do right by it, whether that means responsibly rehoming it or letting it live out it's days as long as it's quality of life is good.
As many forum people know, I threw in the towel last year, just couldn't handle putting foals on the ground and not knowing what their fate may be down the road. I feel blessed to have placed all of the horses I had for sale in less than a year, and I was able to do so privately, and I did keep two. My awesome senior stallion, who was gelded last fall at the age of almost 17, and a companion for him, my ditzy taller mare, she is a gem. I felt my boy Omega, the Buck Echo son in my avatar, deserved to live out his days and I owed it to him. Logically at his age, someone would have wanted him him for a song and a dance to use as a stud for a year or two,
then what, decide since they really didn't pay much for him that he has little value and dump him at a sale, No Thanks!