The thing is with such a large investment you simply cannot jump into the bandwagon and promise a network that doesn't even have sufficient infastructure and resources to promise it's success.
If I am not mistaken, I think LTE is also backward compatible with HSPA/HSPA+ devices as well, so customers who want LTE on their device will simply need to buy a new phone that supports it this removes the headache of transitioning a entire market over to a new technology such as wimax where it simply becomes a customer headache such as Sprint did with iDEN and CDMA, complete disaster.
LTE will more than likely be only available in the major cities, where it will be used to increase the capacity of cell sites which only means more customers in the same area at a given time without the need to add additional towers.
I am sure T-mobile knows exactly what they are doing, and if MetroPCS transitions from CDMA to LTE fully with T-mobile as a roaming partner, I can see T-mobile and MetroPCS be a force to recon with and AT&T and Verizon have something coming that's going to hurt their pockets unless they already have something up their sleeves.
Way more Wimax then Tmobile 3G around here. The real difference is when I don't have a Wimax connection, my data drops to about 1000kpbs 3G on Sprint, compared to 50-150kpbs Edge on Tmobile. I think you could call Tmobile a NICHE carrier, LOL. They just need to STFU and not bring attention to themselves. They might end up on a 3G coverage commercial, that would be hilarious.