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The issue is the scale of your individual impact. The pacific crest trail is a single-track footpath that travels from mexico and provides recreation for likely 50,000 plus (total guess) people every year. There are also approximately 300 people every year that spend about five months hiking the entire length of the trail. I think that it is completely reasonable to say that the aesthetic and ecological impact of the entire pacific crest trail system is less than that of even a few hundred hundred miles of off-road trails, and hiking definitely has a much lower impact on an individual basis.
p.s. the pacific crest trail is not much larger than an animal trail, and areas that have significant erosion are due to pack-animals, which I also do not think are appropriate in sensitive areas.