I hate to burst the bubble nastrodomos but I have yet to see a peer-reviewed study or one with solid methods that provides evidence that backpacking or hiking on a whole has decreased. Industry studies are notoriously subjective with potential customers usually being the population of interest, which leaves out many people from the sample.
In the only nation-wide study on recreation that properly samples the US population (National Survey on Recreation and the Environment or NSRE for short), conducted by the Forest Service's Research Station in Athens, GA. It found that recreational walking has increased more than twofold up to 2003 and backpacking saw almost a 100% increase, even when compared to early 80's data. I doubt that less people are hiking, but they might be hiking more places (the AT is old hat for a lot of people and they are seeking more "out there" experiences, that no doubt maintain less data on their use).
Many park numbers have gone down in the past decade, but not because there are actually less users. It's because they used inadequate methods that systematically counted more than what was actually coming (like the practice of counting a visitor upon arrival AND departure, which doubles the figure in one swoop). The NSRE was created to provide a single methodology for areas since most data didn't synch and it was impossible to compare or aggregate with all the inconsistencies.
So maybe less people are simply avoiding Amicalola. i know I didn't sign it on my thru. Maybe more people are simply going straight to Springer, because sites like this tell you how to do it when before it was more mysterious.
I do know one thing for certain. Until there is a survey that adequately counts on the AT and other trails, all we do for now is speculate. Which is fun.