When I lived in Ohio, there were fewer off-road riding areas than there are, nowadays. Withing 20 miles of Columbus, the mountain bike riding was scarce.
So, I would load up and drive down to Chillicothe, every now and then, to ride on the Off-Road Vehicle Area trails. The ORVA trails were located in a wooded area with some wicked steep climbs , and quite a few creeks and creek crossings.
One day, in the spring of the year, I went down to the ORVA and unloaded the bike. It had rained, the night before, and the jeep trail leading into the woods was a little muddy. I had only been to the trails a couple of times, in the winter, so I didn't know what awaited me.
I climbed the first hill, and then headed down the other side to where some single track trail cut off from the jeep road. As I dropped down to the creekside on the trail, I noticed that the mud was getting gummier and gummier. Eventually, I had to stop and use a stick to clear the mud from between the wheels and the frame, so that the bike would roll.
I rode about 10 miles, that day, and I was totally worn out by the time I got back to the truck. And, I wasn't the only thing that was worn out.
My brake pads, which I had just replaced, were worn down to the metal. I was stunned.
As time went by, I came to expect that. I actually started carrying extra brake pads with me, when I rode there, just in case the pads on the bike wore down before I got back to the truck.
Some of the guys in the bike shop said that they had stopped riding the ORVA trails, because they not only went through a set of brake pads in a single ride, but they would also wear through the rims on the wheels within a single season of riding.
The geology in the area of those trails is pretty interesting. There is a sandstone layer which overlays a shale, which is exposed on a lot of the hillsides, due to the erosion which formed the landscape. When the rock further erodes, the sand from the sandstone mixes with the clay articles of the shale and forms a natural grinding compund.
The rubber brake pads and relatively soft aluminum of the rims didn't stand a chance. The main advantage that disc brakes have, for a mountain bike, is that they are not as susceptible to the sort of wear that the old-school components suffered, in the conditions at the ORVA.
Discs aren't necessarily any more powerful than a well-adjusted V-Brake. They just work better under adverse conditions.
Disc brakes were not really available, in a commercial sense, for bicycles in the late 1980s/early 1990s. The technology just wasn't there, at the time. Had they been, I would have been an early-adopter of them, for sure.