Seems I've read interest in backpacking has actually declined since the 70s and 80s, although this may be as percentage of population, rather than absolute numbers.
Regardless, in my view the primary importance of wilderness isn't scenic beauty (in our eyes) or opportunities for (human) solitude. I love those aspects of wilderness, but the way large tracts of roadless wilderness work as functioning ecologies is a lot more important (wildlife migration routes, plant and animal habitat, clean watersheds, etc).
What we really need are *more* wild lands and roadless areas, and we should be pushing for that, rather than (implicity) accepting the wilderness we have now is all we're ever going to get, and then trying to ration human traffic there.
We should be looking at abandoned rural and rural/industrial land (eg, areas once used for mining, power production, logging, ranching etc) as candidates for new roadless wilderness. Here in the West, there are a lot of small towns that won't exist in a few decades, and extractive industries that have declined and will continue to decline. Wilderness areas created from former settlement won't be the same as that enshrined in the Wilderness Act of 1964 ("...an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain..") but that ideal was largely a myth anyway.
One of the wilderness areas I'm fond of (Hells Canyon) had a much larger human population 50 or 100 years ago than today. The remnants of past settlement are an interesting part of the human experience backpacking there. Plenty of other areas have the same potential.
Dave Foreman's _Rewilding North America_ lays out a pretty interesting vision for the future of wilderness, and the necessity for functioning and connected ecosystems, not just scenic parkland. Roderick Nash's _Wilderness and the American Mind_ is also very good, particularly for showing how American attitudes toward wilderness have changed over the past couple hundred years.