Most of the route is through extremely rugged mountainous terrain, not tundra, and developing a roadbed that can survive -70 F winters is a lot more involved than "piling dirt into a shape"

I traveled it before was paved, and it was, and is, a true engineering marvel.

Yes, I both unravelled my vast illiteracy on road engineering in general and understated a bit on purpose. My point was that there is not much existing infrastructure to take into account up there. Your point taken though.

And yet the infrastructure cost of rural areas is regularly cited as a problem, and moving people into big dense cities is proposed as the solution.

One meter of road in the middle of san francisco would be used more than a meter of road in rural areas.

While the costs are higher- so are the benefits

The benefits per dollar or per meter? How many passengers are traveling on a bus on Van Ness in a given rush hour?