I've hit Mr. Legere up quite a few times on Twitter for praise/funnies/help and he tries, but honestly he is so in bed with Apple and Samsung, it's hard to think that MS/Nokia is even on his radar. For better or worse, T-Mobile turns Windows Phones fast. With having the 530 and the 635 in their inventory - and both available under $100 without contract - I don't see them committing to another Windows Phone until market share picks up. This would include devoting the resources to updating the 925. They abandoned the 810, I expect the same with the 925.
That said, I also don't see T-Mo bringing in another flagship Windows Phone. I think they sold a crap ton of the 521's for $50-100 and that is their bread and butter price range right now. Smartphone users that want that $100 a month family plan, but don't want to buy a phone. That sub $100 price point is great for those users. Right now, you can snag a bevy of CPO devices in that price range or a few lower end Android phones. The Alcatel OneTouch Fierce 2, Evolve 2, the LG L90 and the Sammy Exhibit are in that $100 range, new without contract. Looking at the high end - Apple, Samsung, LG, HTC and Sony are top tier devices with Apple and Sammy pulling down first row mentions.
I just don't see them wanting to push Windows Phone until Window Phone can compete market share wise.