> If you consider the small percentage of acres that are actually walkable by humans

Grand Canyon was an extreme example, but people can go over rough terrain. That’s a big part of what being in nature means.

In terms of damage to nature, any large animal is going to trample plants, even deer trails very noticeably impact plants let alone a grizzly or moose. That’s just part of nature, it’s scale that’s the issue. Hiking trails are just points of concentrated damage which is one viable option.

> They get there from feeder trails and are constrained by what is traversable in the first place. And who gets to choose who walks off path? Everyone gets to?

Everyone gets to as long as they are in a low density area and don’t pick to exit at some interesting spot. You take a trail length divide by the number of people roughly using a trail, and as long as you get to ~1 person/foot per 50 years it’s fine to exit in a mathematically random spot not just because you pick what feels random. Obviously this excludes a great number of trails people frequent, but that’s kind of the point.

Now you should also avoid interesting destinations and be careful how you come back. Anyway, there you have it a scaleable rule that works for hundreds of millions of visitors without creating undo harm.

> Off-trail trampling causes millions in damage to national parks:

Did you read what you linked? That damage is explicitly mentioned as: “sinks ripped off restroom walls, road signs mowed down by some hooligan's pickup, spray-painted graffiti on roadside markers”