Two years ago I would not have speculated about UMTS being abandoned anytime soon. LTE was still relatively new and in Europe, UMTS was the main air interface technology for high speed mobile Internet access. But things have changed rapidly and LTE has taken over that role by storm. Today, most high-end and even mid-range smartphones come with LTE support and in many countries, LTE is deployed in a large coverage area and with bandwidths far surpassing UMTS. UMTS might still have the coverage edge for now but in some countries LTE is available in many rural areas where UMTS is not due to it's use of the 800 MHz digital dividend frequency band. All of this makes me wonder if the many new UMTS features that were standardized in 3GPP Release 8 to 12 will ever be used in live networks!?
Those that I can think of that would provide some value for example are a faster high speed uplink beyond the two to four Mbit/s that can be observed today, putting the slow FACH on a common E-DCH and lots of other small enhancements on the protocol layer to make the system more efficient and to have more users active per base station simultaneously. But with LTE going strong I have my doubts if the time and effort will be spent. After all, network traffic shifts towards LTE and UMTS is rapidly becoming the fallback solution in the areas where LTE is not (yet) available and of course for simultaneous voice and data during CS-Fallback while VoLTE keeps having teething problems.
So I wouldn't be surprised if in another two years LTE coverage might have reached the same coverage level as UMTS and the GSM/UMTS-only legacy device pool has shrunken considerably. Perhaps even VoLTE has made it into the real world by then and can really serve as a circuit switched voice replacement. But VoLTE has been in the making for so long, so I'd say this is the big unknown but a must have for any strategy to ramp down UMTS, as falling back to GSM for a voice call and then not having Internet access during the call is just out of the question.