I would agree that there are practical limits to slowing down. As I pointed out in another part of this thread, I can't go slower than about 3.5 mph. I'd really up that to 4 mph for really appreciable distances. And when I'm climbing and hit first gear, there's no place to go but to slow down or push harder or walk. That changes everything.

But as I also mentioned on the original post, my weight vs. speed questions that started this thread are not meant to address the boundary conditions. There's a lot of room to go when increasing a load beyond 20 pounds, before you reach the 120 pound mark (as an example of a load that pushes practical limits on certain terrain). It is this domain that has been my focus.

I recently took a three week trip loaded with 80 lbs. of gear. I had a few long 7% grade climbs where I found first gear and I hugged the lower limit of my speed (3.5 mph). There were some steeper but shorter climbs where I walked. I got by with about 300 yards of walking during 1200 miles of touring. I probably could have avoided walking at all if my load had been half as much. But the trip was very practical to me - I don't mind walking that short distance to have the benefit I received from covering contingencies and/or personally enjoyable items.