CDMA voice is part of Sprint's long term vision. It's not going anywhere for a while. One of the main reasons is that it can provide HD Voice quality on a sliver of spectrum compared to gsm, which is clogging the pipes for ATT/T-Mobile right now.
Also, Sprint really needs to finish Network Vision, with the 800 mhz LTE deploy, to see exactly what their LTE footprint looks like. They need to make sure that coverage would be ubiquitous enough to offer voice over LTE, and be able to drop back to CDMA if coverage is not there. They don't have any LTE roaming partnerships with other national carriers and LTE doesn't travel as far as CDMA 1x by a non trivial distance. I would like to see them wait on VoLTE, until Verizon has worked out all the bugs.