A lot of T-Mobile iPhone users will be happy w/ the 3G re-farm. But since it won't be using their AWS 4G HSPA network, it will keep a lot of current customers happy too!
Its still affecting current customers because to accommodate the change they are adjusting the layout of current frequencies. Remember, 1900Mhz is currently used for voice service.
In 5 years T-Mo will be using 1700Mhz almost solely for LTE, and 1900 will be the UMTS frequency. This will be a much better layout than they have today for phone compatibility. 850/1900 for voice, 1900 for UMTS, 1700 for LTE. They'll be missing UMTS850 and LTE700 that ATT uses, but at least their phones will be pretty much cross-compatible.
But moving forward T-Mo is going to need a longer-ranged/better-penetrating frequency for LTE, and as far as I know their only option without buying new spectrum will be 850Mhz (on the other networks this is filled by 700Mhz and 800Mhz LTE bands). Not the worst idea, but moving forward 850 is their primary voice frequency band (shared with 1900 today) and T-Mo has already been burned (with AWS) with phone selection when they are the only carrier using a band.