Someday, maybe in a couple hundred years, maybe a couple thousand years, maybe far sooner than that, we will be going back to the 19th Century.
The oil will run out, as will the copper and other metals required to replace fossil fuels with renewable energy. We will need horses, oxen, and other large animals for transportation, planting, and harvesting.
As f***ed up as it is, horse racing does offer the incentive to keep a critical species healthy and viable. As to the need or desirability of the horse slaughter plant - We anthropomorphize horses so much that it IS emotionally harder to accept the commercial slaughter of a horse than a nameless chicken. A bull in the field is usually just a bull in the field, while Maddie's Joy is a 5 year old chestnut mare who LOVES some pancake syrup with her carrots and oats.