It has become apparent that Sprint doesn't have any intention on improving their network or enabling VoLTE instead of using voice-only CDMA for calls.
Not implementing VoLTE causes a variety of problems. While on a call (or even while your phone is merely ringing for an incoming call), Apps that are dependent on background data can not function. Messaging using the RCS protocol (an eventual replacement for SMS) will also not be operable. Battery life on CDMA is significantly less than their GSM/UMTS counterparts. Finally, the sound quality over CDMA especially when the voice data is ultra-compressed is horrible. The cross-conversion of passing voice traffic from the public switched telephone network back to IP telephony systems is well, inefficient and substandard.
I've been holding out on this for too long, and with the T-Mobile merger thing, I don't see Sprint making any changes to this anytime soon.