There are still wild horse within 100 miles of where I am, and yes I agree it's only been since the 1600's or so for domestic horses, but it was still a historical way of travel on our lands. I personally don't like how horses tear things up, but it doesn't bother me enough to complain and perhaps people that would not get a chance to enjoy the wilderness on foot, get to enjoy it without an ATV so I'm cool with it. A herd of Elk will tear things up as bad if not worse than a couple horses period. I've also seen bear scat, every 50 feet for a couple miles on a mountain side. Face it, there's crap in the woods, in the real woods.
As for leashes, I don't think we are doing the animals a favor. Almost all animals have predators (deer for example), to leash the dogs up so they do not chase wildlife is to make the animals think a dog is not harmful to them providing a false sense of security to the animal. It's no different than if we walked in the woods, and all predators were on a leash, we would have a false sense of security. Now, I don't agree that dogs should just chase anything, but a well behaved dog is fine off leash. I've sat an observed 50 head of Elk from 100 yards with my dogs at my side.
If you crave more solitude, less horses or less dogs, it's pretty easy to find just get off the main trails, bushwhack, follow game trails etc. When I'm out, and feeling non social, I just go off trail. My dogs have always been fine, following me over whatever mountain, canyons or off trail route I decide to go. Yes they step on my snowshoes when I'm breaking trail in deep snow, but they are happy to go and never show any signs of distress. To say a dog can't do things a human can, is silly. Sure certain breeds are better suited for certain things, but almost any good mountain dog is pretty capable with a little bit of break in period.
We can agree to disagree, but I stand by my thoughts on the subject.