Yes, it costs money to build out last-mile infrastructure. But even in very low density areas, it can pay off. Recurring revenues of $50-100/month per customer can in fact pay for the initial investment. Especially if the investment was a wifi tower. It's the kind of thing one motivated local can pull off.
Building a worldwide network of peers who all respect NN is much much more difficult. Even if you could set up some organization like EFF to define some "Bill of Rights" and then certify conformant ISPs with a cool logo of approval, customers will not choose ISPs based on that logo. Average American consumers will not demand NN.
So your local ISP--even a well-intentioned one--will do as you suggest: "Simply purchase transit bandwidth from the big guys." There ends NN.