Small, rural carriers (there are dozens of them) would benefit as well, as the gadget makers could contract with these small companies to extend the reach of their devices to gain a competitive edge.
Device makers would be in the position to squeeze carriers on costs, because they would enjoy economies of scale (rather than each consumer negotiating terms alone against the carriers).
In the gadget business, simplicity sells. While carriers are rewarded for complexity, device makers are rewarded for simplicity. This fact would encourage device makers to offer affordable unlimited plans. So, for example, you would buy a gadget for the full price, then pay a reasonable monthly fee for as long as you use the wireless data service -- no contracts. Consumers would be able to compare offerings, and make rational choices.
Best of all, success in the carrier racket would require providing better service, lowering prices and maximizing coverage areas, rather than shell games, "gotcha" pricing and a business model based on customer confusion.