T-Mobile has been having a fine job of bleeding Verizon and AT&T of subscribers.[1]
Keep in mind, T-Mobile also just won a ton of 600mhz spectrum that will allow them to plug a great deal of the underserved rural areas without having to densify the network. First bits of that should be online at the end of the year.
CapEx is where T-Mobile is putting its money, not buying satellite companies (DirecTV), launching a failed mobile television service (go90?), or buying crappy mexican carriers (mexico nextel/lusacell).
If T-Mobile were to merge with someone to their benefit, it would be Dish as they have a nationwide AWS-3 license and won a good chunk of 600mhz as well. They could also start to pull the bundle shit AT&T/Verizon do.
