It is just not that hard to avoid crowds, even in tourist magnets like Yosemite. Avoid the popular spots and there are lots of open trails. There are a multitude of lesser known areas that just don't get a lot of traffic. Right next to the 16 million or so living in southern California is Channel Islands National Park, which is typically the third or so least visited NP in the system. Of those who visit, only a small fraction ever get on trails and venture any distance from campgrounds. This same story can be told in many areas throughout the west. Most people want to go where everyone else is going – Yosemite Valley, Grand Canyon, Yellowstone.
Years ago, the trails were really deserted, and they have become more crowded recently, but there is still lots of unfrequented country. If all else fails, go in the middle of the week (get a job that requires you to work weekends) and observe how the trails depopulate.
