In a situation like we're discussing, there are too many variables for anyone person to point definitively to a causitive link.
However, in the time I've been actively involved in the horse business as an adult (30+ years) there have been several economic recessions. (And gas prices during the 1st Gulf War hurt the horse business because of the cost of hauling.) At no time in the last 30 odd years (40 if you want to count my years as a junior as valid and relevant) has the market for a basic pleasure horse or beginner's horse been this bad. And that's a sentiment I hear repeated over and over again from professionals, vets, farriers and rescue organizations. The unique factor in the current situation is the closure of the US slaughter plants. So I chose to believe those things are related. Is the general state of the economy a factor as well? Of course. Does the price of hay affect the cost of horses at the bottom of the market? Sure, of course it does. Backyard owners decide they can no longer afford the expense of a pet or hobby horse, and professionals no longer take resale prospects on as readily because the value proposition has changed. But again, hay prices have fluctuated over the past 30 - 40 years as well, yet the market now is worse than it was during a drought year.
I could perhaps believe that today's market is a result of a "perfect storm" of the economy, hay prices and the glut of horses at the bottom of the market caused in part by the slaughter situation.