I was a NOLS instructor from 1969-1975 and can really relate to your experiences! As much as I also have gone to lighter packs, I think for the general backpacking public, we have to acknowledge that a lot of people still carry packs of about 30-40 pounds. Perhaps you need to mention that crossings are a lot easier with a lighter pack. This may entice them to lighten up! I see a lot of external frame packs heavily loaded when I am out so I would not discount them as something that has to be handled when crossing a stream. I now hike in light-hikers and they dry quickly unlike the leather boots of the past. For a few crossings per trip, I just use the shoes. IF I anticipate several crossings per day, then I bring wading shoes.