I agree with you that it makes no difference - to an extent - from a horse’s perspective what the cause of pain or discomfort is. Although I would also argue that a horse, being acutely attuned to the herd, can sense threat and danger and knows whether its owner is trustworthy or not and whether or not its environment is safe. And that will in turn affect its stress levels and its propensity to suffer from stress related behaviours (such as crib biting) or illness (gastric ulcers).
If you are a well intentioned but ignorant owner or rider, as you say, you can improve and become more knowledgeable ie there is hope that the horse’s treatment will improve and in many cases it does, as we all start from a place of ignorance don’t we?
In the case of someone like CD, who we now know has malign intentions, the reverse is true. She has all the knowledge in the world and no intention of using it for good. That leads to a hopeless position for the horse.
No one is suggesting that we shouldn’t all make efforts to improve horse welfare and eliminate bad practice wherever it comes from , at every level of equestrianism. But someone with malign intentions will not be motivated to do that. Whereas someone who over-rugs their horse overnight because they are afraid they will be cold, will be horrified to find it sweating in the stable the next morning and make adjustments. So intentions do matter.
I’d also argue that, in addition to the many problematic issues you raise, horse welfare among amateurs is improving in many areas too, as we learn more about herd interaction, behavioural science, over-feeding and it’s connection to laminitis, different ways of keeping horses such as paradise tracks, natural land management, bitless riding, the bare foot movement and the many horse- centred training programmes available. Not many of these things were discussed forty years ago when I first started riding.
Even when these practices are implemented incorrectly, at least an effort is being made to study sleep and grazing patterns , communication signals etc and many owners are trying to accommodate the three fs and manage horses in ways that mimic their natural environment and suit their characteristics.
(Btw, I’m a middle-aged woman too, I have even been accused of being a hag with a flag 😂 as women form the majority of amateur riders, surely it’s only logical that they should take credit for participating in the above positive changes too, as well as the instances of poor riding you accuse them of? Poor riding definitely exists and some riders are unfit, unbalanced and too heavy, but if their intentions are good, they will learn to do better. )