I doubt GSM is going to go away on the handset for at least 10 years, simply because there is so much GSM deployed world-wide and it will be convenient to be able to make voice calls even when visiting the most rural parts of the developing world. Given software radio and DSP technologies, there is very little extra cost to supporting GSM alongside UMTS and LTE.
GSM also has good coverage for rural areas, as a GSM cell can reach up to 35 km (http://en.wikipedia.org/wiki/GSM#Cellular_radio_network ) vs. "over 10km" in theory for UMTS (http://books.google.co.uk/books?id=7m-MnwW_o7AC&lpg=P... ). Even today in a Western European country, the only way I can get UMTS 3G coverage at home is via femtocell, and I'm only 5 km from the nearest 3G base station. The economics of deploying a UMTS or LTE base station closer to me will only stack up if there's a very cheap wireless backhaul technology that can handle the required throughput for multiple 3G/4G subscribers.