As of hour 16, I have finally upgraded all the buildings and unlocked all the recipes, so I can at last let my poor, overworked horses rest. I have to say, I do not care for the way illness works in this game. Sending a sick horse out on missions is slightly more efficient than sending a healthy one, because their performance is exactly the same, but since you can't sell a sick horse, there are fewer missions to scroll through to get to the most profitable ones, and there's no risk of selling it accidentally (which is something that actually happened to me - luckily it was one who had already bred, and was thus useless).
It kind of makes me sad that the game rewards you for not healing your horses (or more accurately, it punishes you for curing them, because the potion costs money, but it is impossible to predict when a horse will get sick, and the only way to be absolutely certain it won't is to not work it at all), because those horses look pretty pathetic marching off to the gates with their heads hanging down. If I had more of a connection to these animals, I might be willing to sacrifice efficiency for kindness, but they are basically just interchangeable money-makers, so fuck 'em.
I also have one of the magic crystals, but as far as I can tell, it does jack shit. Maybe something will happen if I get all five. Unfortunately, breeding a level 5 horse is so time-intensive that it will almost certainly never happen. I'm on the cusp of getting a second one, but by my estimates, I don't think I'll have time to get the third.
I suppose I could continue until I get all five horses, and thus discover the secret of the magic crystals, and a (small) part of me is conflicted about leaving the project half done, but I'm positive that whatever it turns out to be won't be worth it, and the sheer amount of repetition and waiting necessary to get there will far outweigh any meager satisfaction I would get from the journey.