Why is GSM still used?
This started as a comment reply to @user10008 but got too long...
Even after the towers are all upgraded, the carriers can't immediately switch off 2g service for a number of reasons. The biggest issue there is that not everyone upgrades their phone frequently; in particular this is true of people who use it as just-a-phone or an emergency-use-only-phone.
There are also commercial/industrial embedded systems that use the cellular network to report home. Low bandwidth devices like monitoring systems, ATMs, or credit card readers have no need for high speed data and no reason for the owners to upgrade frequently. AFAIK they typically have SLAs with carriers to guarantee longer term operation than the standard 2 year consumer contract.
Regulatory issues can also be involved. In the US, you're not allowed to turn off a voice network without giving customers free new phones without contract extensions. As a result, the major US carriers have scheduled 2017-21 sunset dates for 2g service to give as much time as possible for the long tail to upgrade on their own. That said, the carriers probably will refarm all but a tiny sliver of their 2g spectrum to 4g (5g?) prior to the shutdown to maximize utilization rates. When the finally do force the issue and shut down their 2g network, they probably will end up having significant numbers of customers who were coasting month to month due to apathy shop around and decide to change carriers.
When Sprint shutdown the iDEN (2g with no upgrade path) network they inherited from the Nextel acquisition; they suffered heavy customer attrition in the final months from people deciding to switch carriers instead of taking a free replacement. IIRC the loss rate for the final few months was >50% of the remainder and several hundred thousand customers in total.
In order to make a cellphone tower UMTS-capable, various hardware upgrades need to be made to it. This costs money. For that reason, many cellphone towers, especially in rural areas, have not been upgraded yet.
As long as there is not near-100% UMTS coverage, cellphones will still need to support a pure GSM connection to ensure that the user has connectivity in areas where no UMTS is available yet.