I wonder how many National Parks will exist for exploration by my grandchildren or great grandchildren. I've already seen so many trails, rivers, or "attractions" closed because some entitled guest (or guests) decided they were allowed to do ____. The end result is a restriction on all visitors. Eventually, I fear the only way to enjoy the old caves, or wind-swept dunes or delicate back country will be to have money,…
Probably most of them; it's more the case that a few of the National Parks are just too overcrowded--Shenandoah and Yosemite probably the worst of the lot. Although some of the National Parks do have issues that aren't related to overvisitation (Hawaii Volcanoes and Everglades both come to mind).
So if you want to see natural beauty, be willing to rough it a bit and go were people don't.
Of course, the most popular places are considered the most "beautiful" but that's not entirely true - and the popular places you can watch documentaries on.