T-Mobile has an incredibly long history of abandoning their Windows Phones... There is no doubt about that. The likelihood is that they will support their current Windows Phones and for a short time after they go EOL.
They may talk about removing pain points for their customers, however you are just a number to them nothing more. So while their prices are good and they've successfully introduced some very positive competition and worthwhile policies into the US's cellular market, they have a long way to go with their customer service practices. I harped on this a lot in the past, but a two year guarantee of updates was part one of my suggestions, and a real exchange of information with their customer was part two. So I really am impressed by HTC moving forward on a two year guarantee of updates for Android, Whether or not HTC was forced to do it by their poor standing at this time, it really doesn't matter; it's a good move for their customers. That's a policy that should be implemented by all carriers.