I realise this is not really an option due to the sheer volume of GSM (only) users, backward compatibility, redundancy and so on, but from a more academic perspective would it be feasible? Say you had a complete greenfields scenario.
So on the cost issue for the equipment vs bandwidth - right now the issue is data only, but if you are using the 15Mhz for your voice revenue as well how does this affect things? If all the operators shared their 900 frequency and there was VOIP prioritisation, would that be more effective (that should get a nice 50Mhz odd block or so surely).
What I am getting at is using the GMS TDM system you only get a little bit of bandwidth which is used exclusively for voice - if you used a packet based system that has QoS for voice built in natively (LTE does as far as I know) surely we could get way better use out of that spectrum. That would mean the end of dropped calls due to networks being busy (but not slow data rates due to congestion).
Kind of like the phasing out of analogue GSM where it was used and refarming that spectrum. Cellular actually use VOIP systems as it is, just they run them of the GSM TDM system instead of the packet data connections. I suppose once VOIP over the existing dat networks gets more pervasive and celluar companies can't tie voice in to the current GSM model it will then become a popular idea.