The problem with the US mobile market isn't a monopoly - it's an oligopoly which is much, much worse. An oligopoly is a market with three or more players all doing the the same thing. No player wants to rock the boat by trying something different so the consumer pays $19.99 for a CD no matter which company made it, gets their choice of crappy 22-minute sitcoms with the same recycled jokes used decades earlier (ABC, CBS, NBC) or the same low-quality, "good enough" car that will last for 100,000 miles if they're lucky (Ford, GM, Chrysler). It's the illusion of competition without the benefits of differentiation or lower costs for the consumer. Just as change forced the oligopolies cited above to adapt, so too will changes in the US mobile market break up the Verizon, AT&T, Sprint, T-Mobile oligopoly.
There's a long list of likely suspects who have the cash and a lot to gain by either starting a new network (Dish) or acquiring an existing player (T-Mobile) and forcing them to play ball. When customers jump ship from one carrier to the other about half choose an iPhone to use on their new network. If Apple were to buy their own network and make the iPhone exclusive to their network they wouldn't have to share revenue with any of the other carriers and they could charge a premium for their service. Their biggest problem would be acquiring enough islands and secure warehouses to store all the cash they would collect. They could also offer things like unlimited wireless data bundled with Apple TV, iPads, etc. either for a monthly fee or bundled with the product. I could definitely see Steve pitching this to the board, but I doubt Tim Cook has the gravitas to make it happen. If the fruit-factory starts turning out less successful devices I could see their board approaching Cook about making it happen to shake things up though.
With a whopping 2% market share it won't be long before US carriers stop taking Microsoft's money to carry their failed Windows Mobile 3.0 devices. This leaves MS with a choice - cut and run (Kin, Zune, Silverlight) or double down by scooping up Nokia and/or HTC and acquiring a network. Even MS knows that mobile devices are the future and without gaining some traction in this segment they have a much darker future. They seem to be trying to copy Apple (mall stores, telling manufacturers what hardware they're allowed to use with the OS, building hardware) so I could definitely see them trying this path if Apple did it first.
Google. They're already bringing high-speed internet to Kansas. By becoming their own carrier they could gather even more data about their customers to sell to marketers. A nationwide wireless network would work nicely with some of their other ventures (self-driving cars, Google glasses, etc.)
While T-Mobile's 3.5G HSPA+ network is nice, T-Mobile has other issues. They have very little coverage outside of urban areas and have very few roaming agreements with other carriers. The best carrier for you will depend on your needs and location but DT hasn't been willing to invest in TMO. True, they're finally starting an LTE network but they have the fewest towers of the major networks and the smallest coverage area overall. To compete and attract NEW customers that they will need to grow they will need to start building towers. DT hasn't even hinted at that. My guess is that they are just putting on a new coat of paint for when Apple, MS or Google come to kick the tires.
The move to separate the phone subsidy from the service essentially makes TMO a MVNO that lets customers pay at the end of the month instead of the beginning. I suspect the move is part of DT's thriftiness - it's a lot cheaper to run a network of you don't have to, "loan" your customer $600 worth of hardware and hope they pay it off over 2 years. I don't see the move catching on because US customers are used to the bundled system they have now, no one is going to want to fork over $699 for a new iPhone, getting used phones is going to get a lot harder since manufacturers are moving to non user-replaceable batteries and the savings aren't that great. TMO's unlimited plan goes from $89.99/mo to $69.99/mo - so you're only saving $20/mo.