Regardless of your personal feelings about slaughter, the fact remains that slaughter facilities in the US closing had a very chilling effect on the US horse market, particularly on the lower end of the market for family/pleasure and trail horses.
I have two horses in my paddock right now. One is a rock solid, dead broke, well educated registered 10 yo QH that is sane and sound and anyone can ride. He also had good feet and is a ridiculously easy keeper. Ten years ago a horse like him would have been anywhere from $2500. - $5000. He was free to me. The owner could have gotten $1500. For him but much preferred to know he was in a good home.
The other is a drop dead good lucking, athletic QH, 9 years old that requires a little riding but is a boatload of fun. Also free to me, and for the same reasons.
It's a tough time to sell a horse.
If a lot of the unsound, unsuitable, badly bred, need more work than they'll be worth horses were taken out of the bottom of the market, my two horses' market value would increase.
I may not like the mechanism by which the glut of unsalable horses will leave the market; but I can't deny the effect it would have on the market, no matter how distasteful I might find it.