T-Mobile to Sunset MetroPCS's CDMA Network by 2015

There's one other part you're missing here.

MetroPCS isn't going directly to VoLTE because it wants to be a pioneer in that technology, rather it didn't have sufficent spectrum prior to this purchase to run both the CDMA voice network and the LTE data network. So their road map was to launch VoLTE migrate voice from the CDMA network to VoLTE, sunset the CDMA network, and use that spectrum to increase their LTE capacity.

Now they have plenty of spectrum. They can now keep voice on GSM/HSPA+ and leave LTE (for now) purely for data.

Long-term they can then migrate to VoLTE (which every carrier will inevitably do) and then expand LTE capacity by using the current GSM/HSPA+ spectrum.

This is a long-term process because of the huge amount of cash needed to completely replace the network with LTE. The idea of this transaction is to get the company sufficiently cash flow positive to be able to fund the conversion to LTE over many years.

This is not a strategy unique to Metro. In time you will see AT&T and Verizon move to VoLTE, sunset their existing 2g and 3g networks, and ultimately be allocating all of their spectrum to LTE. At least until something newer and faster comes along that will gradually begin to replace LTE. This will likely occur before their networks are completely converted to LTE, but such is life in the wireless sector. 2G and 3G will be going away in time, just like AMPS.

By Metro expanding into T-Mobile markets, they would likely either be selling all prepaid as Metro and all postpaid as T-Mobile, or they will operate Metro as a carrier owned MVNO as Sprint does with Virgin and Boost.