Sorry this is a long one!
I personally agree with everyone who has said that the 2 disciplines are separate but equal.
Movements in western riding such as the slow-n-low are meant to be used on horses with the proper training and conditioning base so as to not stress the animal, and often you Will see horses in the pasture walking slowly with their heads down in a very relaxed position, of course I realize that this is not exactly like the s-n-l, or level topline training but it isn't THAT artificial.
Also people seem to be saying English movements such as collection or jumping are overly stressful on the horse, but as I said these movements are meant to be used with a horse that is properly conditioned to do them. Look at any exited or just playful happy horse out in the pasture and you will see a collected "upward" gait as it prances around its herdmates. Just like in the western discipline this IS an artificial gait used during riding for the purpose of enhancing the performance and safety/comfort of that horse and rider in their chosen discipline.
Finally I think that riding a horse at all is hard on it. It isnt natural to the animal, foals arent born with people on their backs
, so the extent of "damage or harshness" is all a matter of opinion.
btw I am a western rider and I love every minute in my large comfy saddle, on my happy healthy horse