I agree the market for such a game seems small.
Nevertheless, millions of people in the North and South America, Australia, Africa have the "foundation" for a pretty nice off-road build -- Jeeps, Hummers, Pickups, some SUVs etc. It would let them arm-chair what their own vehicles could do in the hands of different owners
A game player would build his vehicle, run the course, survive without needing to make repairs so he has the money to buy more parts. Of course the trails would become increasingly treacherous. And of course, if you rolled bad off a "cliff" or rock you'd be back to zero in the game. Kind of like building a house of cards.
I would antipate that the trails would be somewhat exaggerated in their hazards
Obviously, though you could include environments based on more well known trails, even naming those levels as such. You could network the game -- you have to winch somebody else up to keep the group together
But I think ultimately you'd get to a point where the game would allow you to literally design trails -- climbs, rocks, mud, etc -- that have no basis in reality. That might be fairly amusing. It could give people ideas for the creation of real-life private trails that incorporate some of these unusual game elements.
Keep in mind though, this game would be about making the trails in one piece. You as the driver would have to manage the risks involved in running all the trails, up through the hardest stuff. If you do, you win, if you don't you lose.
I can already visualize the marketing of the game: Real life video images of rock-crawling in the South West U.S., Mexico, Central or South America. Then clips of your vehicle in the game.
Could include a code that would open a scene in which Jeeps chase down litigious environmentalists. That'd be the bonus round.