"Skis are machines invented to make human travel easier and faster. The categorical side of this question seems pretty basic."
ma·chine
/məˈSHēn/
noun
1. an apparatus using or applying mechanical power and having several parts, each with a definite function and together performing a particular task.
"Tom (and anyone else whose mind might not be made on this point), I'd encourage you to think of the John Muir who climbed ponderosa pines to get closer to lightning storms, and bivvied in the steam vents on Shasta. I'd encourage you to think of the TR who rode a hundred miles a day for weeks straight hunting bison and Grizzlies while he was running Elkhorn, or who went on a poorly conceived, nearly fatal decent on an unmapped Amazon tributary after his presidency. I'd encourage you to think of the Bob Marshall who did 50+ mile dayhikes for no particular reason, and nearly died running a rain swollen stream in the Brooks Range. Or Ed Abbey (gumby hiker that he was) nearly getting himself stuck in a slot tributary of Havasu Creek."
Gee, Dave, you come across like you're the only one who has read about the exploits of these giants of the preservation movement. Puhleeeze, enough already. Note that none of those exploits involved mechanical means of transportation, and only horses left any discernible trace of their having been in a location.
"There's a pretty robust correlation between being one of the leading intellectual contributors to how we think about wilderness today and wanting to have a regular and as visceral as possible intimacy with said place."
On this I agree with you 100%. But the unexplained logic underlying your conclusion that: "In my mind it's axiomatic that adding gravity-powered locomotion to foot travel facilitates this." totally escapes me, to the point that I will state unequivocally that, IMO, you are just flat out wrong. MTB's are not gravity powered as their primary means of supplying the energy for forward motion, impose a distance between the rider and the terrain over which he/she travels by separating them from direct contact with the terrain, typically allow the rider to move at an unnatural speed at odds with the rhythm of natural life, and cause unacceptable damage to the surfaces over which they travel.
Wilderness Act text and related comments deleted. Andrew has already covered this one.