He argues that the major problem is that the parks are too easy to visit. If you make people get out of their cars and walk a bit, you might stand to preserve some kind of semi-natural experience. If you pave a road to the rim of the Grand Canyon, it shouldn't come as a surprise that people are going to drive right up to it, get out, take a picture, and drive on to the next overlook. As I'm not going to do his words or argument justice, I'd encourage anybody interested in this to pick up a copy of Desert Solitaire.
If you do, and you find his argument convincing, your next task is to figure out how to sustain the communities that have come to depend on tourist revenue. I'm not talking about the seasonal folks that work for the NPS or a concessionaire, I mean the people who actually live in these places year-round.
You should know that Abbey is not writing from a disinterested viewpoint. He wrote The Monkey Wrench Gang, after all. Still, I find his words both lucid and prescient.