Wow, they must have added that very recently, because I've been checking and haven't seen the 650 listed until now. Secondly, in all the announcements and news I've never seen them mention it releasing in the US. Even still, neither of those supports T-Mobile's LTE band 12. How unfortunate... I guess when the time comes I'll have to switch carriers... Hopefully something changes before then. I really don't want to leave T-Mobile.
I've also looked at the Alcatel OneTouch Fierce XL, while not as stylish as a Lumia, it is still windows 10 phone for T-Mobile, but the phone is pretty large. Maybe there will be a smaller model soon.
Edit: Oh wow, I just saw the 550 was added to their list too, and this one does support band 12. I don't know when these were added to their US website, but I sure am thankful. And even though the 550 isn't really an upgrade over the 640 that at least tells me that Microsoft hasn't given up selling their phones in the US after all, which is definitely good for the future.