Nobody has mentioned that over the same period skier and snowboarder visits are way up after years of declines. Snowshoeing is also booming (according to one person I spoke with the Tubbs/Atlas line is the most profitable of all of K2 Sports areas). There are two things at work with snowsports: first, newer equipment has made it easier for boomers to return to the sport and to continue skiing at older ages; second, the younger generation has been drawn to the "extreme" appeal of skiing/boarding. Taken together, you have something for the whole family... easy turning skis and groomers for the boomer parents and "extreme" terrain (glades, park, pipe, etc) for the younger set.
For all of the technical advances in outdoor fabrics, equipment, etc. sleeping on the ground is still sleeping on the ground. I don't think this appeals to the aging backs of the majority of boomers. Also, I think young people don't find backpacking, hiking, camping provides the adrenaline rush that they seem to require in their outdoor pursuits.
Frankly, I think that National Parks have been too "tamed" to attract younger adventure seekers (the under-30 crowd) in large numbers. With paved walkways, railing, flush toilets, numerous interprative sites, whole villages of overpriced lousy pizza, etc. I think many of the large NP's like Yosemite feel safe, controlled, and antiseptic. Take the snowboarding teen out of the terrain park and bring them to Curry Village in Yosemite and I'm willing to bet you'll hear them complain of being bored, listening to their IPods, etc. Take them on a bushwhack through the backcountry to remote ponds, jagged peaks, etc. and I bet you get a bit more of the adrenaline and less of the slack-jawed boredom of Curry Village.
The trouble is that the boomer parents are far too out of shape and uninterested in true backcountry adventure to take their kids far into the backcountry.
I think the problem, if you want to call it a problem, of decreasing visitation needs to be addressed by a two-pronged approach. First, do a better job of marketing the existing high-end lodge properties and improve the 4-star experience for the salmon and cabernet crowd. You don't need to expand existing facilities, just upgrade the experience at places like the Awanahee Hotel to a true world-class 4 star experience. Give the boomers wine tasting and guided hikes and the almond crusted salmon they apparently want.
At the same time, encourage visitation to the backcountry by highlightingthe vast areas that are more than a quarter mile from the road. Highlight backcountry ski touring in Yellowstone, write about ice climbing in the Tetons, big wall climbing in Yosemite, multi-day backcoutry adventure in RMNP. If you look at the NP literature it is 99% dominated by attractions within 100 yards of the main park roads. They can do a better job of highlighting the adventure available away from the roads, lodges, and valley floors to the thrill seeking, increasingly wealthy and childless, 25-35 demograhpic. I almost never rely on the official NPS literature when planning my own trips to the backcountry in the NP's. I have to wade through various guidebooks and websites to plan my trips. Market the parks to the outdoor adventure set rather than to the non-existent, 1950's Leave it to Beaver nuclear family.
I think both of these can be done without detracting from the conservation mission of the NPS. Neither require extensive, or really any, changes to existing regulations. In fact, I think a strong argument can be made for scaling back on human encroachment within the parks and allowing many place to revert to a "wilder" state than currently exists.
I want people visiting our National Parks. I'm afraid that it will be increasingly difficult to set aside more land for recreation/conservation if the other side can argue, "why bother creating a new park if people aren't visiting the existing parks?" It's in the best interest of all conservation or outdoor recreationally minded people to see visitation increase and for more people to experience the natural treasures that are entrusted to public stewarship.