What did John Muir fight for?

The preservation of nature.

John of the Mountains" was the phrase which is used by John Muir who was the prominent Scottish-American biologist, poet, environmental scholar, glaciologist, and pioneering advocate for the conservation of wastelands in the United States of America.

If there was no John Muir and his works, we presumably would not possess Yosemite National Park. He was also concerned in the formulation of the Petrified Forest, and Mt. Rainier National Parks.

The preservation of nature. He is the reason we have national park.