Hmm, that depends
My guess is that 3G will shrink dramatically, as it's not very good for data and won't work well with LTE. IoT devices currently are overwhelmingly on GSM, and Internet will quickly move to LTE as soon as operators open their networks to the general public.
GSM will still be around in the long term as it's seen as infrastructure. It also can work easily with LTE, so you can have a combined LTE GSM base station sharing the same frequency. And that might in fact be used for voice in the long run.
If people get VoLTE to run on a larger scale, it'll probably enslave humanity and send back a robot from the future to kill the mother of the leader of the anti VoLTE resistance, or it'll just crash and burn and slowly disappear from marketing brochures.