Re: customers are die-hard loyal to a specific carrier.
Maybe, but probably not. In the case of T-Mobile, certainly not.
Case 1 for loyalty: Carrier has some sort of customer loyalty program that gives them more rapid or less costly phone upgrades with a sustained contract. Or possibly a contract that has a free replacement phone clause if the phone is broken or stolen. In this case, not so much a case of customer loyalty to brand as customer getting the best deal, but the marketing twits will still read it as customer loyalty.
Case 2: Punter lives in an area where only 1 or 2 carriers provide reliable coverage. Here in DC it use to be that if you wanted cell coverage in the underground Metro tunnels (light rail) you had to be with AT&T (IIRC otherwise it was Verizon, main point being only 1 carrier had relays). In theory all of the carriers are supposed to permit other cells to call on the network if you are roaming, in practice, not so much. There may be decent technical reasons for the 'not so much' but from the punters point of view it all that matters is his expensive cell phone is a useless brick.
As far as I can tell, T-Mobile doesn't fall into either of these categories. I hear people around here talk about AT&T, Verizon, or maybe Sprint if hard pressed. Never T-Mobile.