I wonder how much those cheap MVNO prices are subsidized by everyone who has a contract directly with the carrier though. Going directly with Verizon or AT&T is super expensive compared to Europe. At the end of the day, someone needs to build & maintain the physical infrastructure. I don't know if the carriers could keep operating if everyone switched to MVNOs, they might not have enough revenue to maintain the infrastructure. North America is pretty large and not densely populated.
That being said, the major carriers here absolutely suck and utilize very scummy business practices that aren't that far off of a payday loan place. Financing an iPhone with Verizon really isn't that far off of payday loan rates, it's a horrible deal.