This is truly not a problem. Auctions still transport horses to slaughter houses outside the US every day, and they did so even when the few (3) American slaughter houses shut down.
70-80% of the American public oppose horse slaughter. We don’t eat horses and we don’t eat our dogs, cats, or even our guinea pigs, and we don’t want them sent to other countries to be eaten. Horse consumption is dropping in other countries. Horse slaughter is 100% predatory business in the US, a business of opportunity and deception. Once horse is taken off the table, so to speak, what then? Horse kill buyers will have to shift their business model to selling horses ALIVE to people who want them. Some already do that as they can actually get more for a live horse, especially one that is trained, than one destined for slaughter, but they have to be willing to take the time to do it. (Most horses that are slaughtered are healthy, sound, young, and have some handling - except all the young, healthy mustangs being dumped once people who adopted them get their incentive check.)
And actually I have spent QUITE a lot of my own person money as well as donations saving perfectly good horses from the slaughter pipeline. Over 70 and counting, and not a single one had any problems.