all carriers are pushing LTE as the radio of choice for voice & data.. phase out could be much sooner than 2021.. perhaps 5 years.. so, handset makers should price their equipment accordingly.. as in across the board price drops for all 2g/3g handsets by 20% a year.. discontinue after 5 years. some equipment can be left in place longer than 5 years if necessary to fulfill contract obligations the way carriers did with analog systems-- but that's it.. I doubt 2g/3g contract obligations extend to 2021..
It's not just handsets (phones) that use the cellular network.
Hence the key phrase CONTRACTUAL OBLIGAIONS... is all-encompasing.. from fcc spectrum agreements to special devices that use the network. Analog cellular was given a 3 year notice of final dismantlement..