"Just because there's no identifiable terminal ideal status doesn't make betterment futile." - @mmshiro
Define "better". Computer speed? That is easy. Effectiveness of a vaccine? That is easy. But in horse sports? Dressage enthusiasts will tell you their sport betters the horse. WP enthusiasts will say the same thing. But would a HORSE agree?
If one looks at rodeo, horses arguably have it better in many rodeo sports than many horse sports. They run fast, stop, and keep a rope tight. They buck for part of a minute. They race forward and their rider jumps off of them and onto a steer - harder on man and steer than horse. I can't help but believe the horse below thinks, "Better you than me, Dude!
If one truly wanted to be as ethical as possible with horses, one ought to make the HORSE feel good and live long. So don't ride, make sure it has ample food, other horses, room to move and exercise if desired, etc. But of course, if that is the standard, most horses would be killed and very few would be left in the world.
A perfect book on riding could be written only by a horse. Only he could easily answer all the questions endlessly argued by us riders. Only a horse could say positively how the rider should sit in order to abuse him less; how his rider should control him so that the aids are easily understood, and how the trainer should school him so that the training proceeds in a comprehensible manner. As long as little pertaining to horses, and hence to riding, can be stated with mathematical precision, riders are bound to disagree...
...Far from all of us, or all of our horses, possess the essentials to become great, and futile attempts to imitate outstanding riders, and to school average horses the way the best are trained, only succeed in making frustrated riders and unhappy animals. On the other hand, everyone who knows his own and his horse's limitations and chooses a suitable form of riding and suitable manner of participating in it, may derive much pleasure from it...
- Common Sense Horsemanship, VS LIttauer, 1963.
Folks can draw lines where they will. It seems to me that rodeo horses have little to complain about compared to many other horses, and the majority of horses have little to complain about compared to life apart from humans.
What bothers me concerning the ethics of horse riding and ownership lies more in how we treat an older horse, or how we treat a horse like Cowboy, who had at least 6 owners in his first 14 years and remains so bitter about arena riding that he still panics at being ridden in one a full 6 years after his lesson horse days. If you could ask a horse, I suspect many would prefer the life of a rodeo horse to that of a lesson horse. But not being a horse, I can only guess.
Thinking out loud: Perhaps horse can only be ethically ridden the way a mule has to be ridden. My mule-loving farrier would probably agree.