"I categorically reject the notion that one bike has more impact than one hiker. This fallacy has lived on too long."
From what I have seen of MTB, it is no fallacy, nor is it to be categorically rejected. Speed, mechanical equipment, and the kind of terrain involved inevitably lead to far more impact that a human being moving at perhaps 2.5 mph on average. The evidence up here in the Cascades, far from over populated SoCal, is far too clear to ignore.
"There is, for example, evidence of legal mountain biking in the Rattlesnake Wilderness north of Missoula before bikes were banned."
Why were they banned?
"It's not in the end about concerns over hiker safety, though that is a factor, but about an intensely subjective judgment concerning what ways of experiencing wilderness are valid. That argument can be made from several sides, but let us not hide behind anything when making it."
On the contrary, it is about hiker safety, as well as trail damage, at least as far as I'm concerned, based on what I have seen and plain common sense. I, for one, am not hiding behind anything, Dave, and am prepared to argue the point on its merits. Speed alone on narrow trails, particularly if the biker is coming downhill on rough terrain and closing on a backpacker going in the same direction makes it a non starter for me, particularly up here in the Cascades, but the same applies in just about any mountain environment I can think of. Even the best of bikers lose control occasionally. What happens when they sooner or later pile into a backpacker from behind on a steep section with a drop off? I could go on, because the scenarios are endless, but it all boils down to an incompatibility in speed at heart, as far as saftey is concerned. At best it will create an atmosphere of unease and eventually hostility that are at odds with why most of us go to the mountains in the first place. This is not to deny the subjective aspect of peaceful coexistance in the wilderness, but that is a separate argument.
"Visitor hours in the national parks have been steadily declining over the last two decades, as population continues to increase. Is infighting amongst human-powered user groups a way to secure a good future for wilderness in a democratic society?"
Declining? In SEKI? Rainier? Yosemite? These places are crawling with people. References?
As far as infighting, the line has to be drawn somewhere, and I think this particular subject is where it gets drawn for me. In a democracy, the majority rules with deference to the minority. To me that means MTBers already have plenty of territory to ride in without exposing backpackers to unnecessary risks and spoiling their subjective enjoyment of wilderness areas. I will abide by the will of the majority, but fight very hard to make sure I am in the majority on this one.
"Today, thanks to technology and attitude, there are hundreds of riders who could go this August and ride 90+% of the trail as it exists from Tuolumne to W. Portal in a leisurely week."
Precisely. In about 1/3 the time most people take to do it. It is a crowded trail with barely enough room for two people to pass each other as it is. What will that be like with bikes going on average 3 times as fast, in terms of backpackers' experience, not to mention the inevitable sideswipes and loss of control incidents on steep downhills? Places like W. Portal, in particular, would be an absolute disaster.