Can National Parks be considered victims of their own success as leisure pursuits?
Yes, this is a common criticism by environmentalists. There is a conflict between true preservation of nature and recreation. The conflict between allowing public access and protecting the resources is because of the Organic Act that established the National Park Service. (http://www.nps.gov/legacy/organic-act.htm)
The Organic Act states: "The service thus established shall promote and regulate the use of the Federal areas known as national parks, monuments, and reservations hereinafter specified by such means and measures as conform to the fundamental purpose of the said parks, monuments, and reservations, which purpose is to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations."
For this reason, parks regulate how visitors may use the park - for example, snowmobiling, hiking trails, driving routes, boating restrictions, etc - to allow them to enjoy the park while protecting the natural features.
Some parks have faced congestion issues due to their popularity. A famous example is Zion National Park (www.nps.gov/zion) which faced pollution and congestion problems from automobile traffic. Today, the park has a bus system that has allowed the same number or more visitors to enjoy the park in a way that is less impactful and more enjoyable. It is a great success story. With more people in parks, the potential for vandalism or defacement of natural features increases, as do problems like trail erosion, or illegal collection of natural, cultural, historic, and geologic items.