I feel like I need to point something about. Maybe it's not applicable to the AT&T network (my info will be manufacturer-dependent) but here goes anyway: a 2G network DOES require OPEX expenditures. You're right about stacks, however this is only valid if when deploying 3G/4G the operator decides to modernize the basestations. Multiband basebands have been in the market for some time, but multiband RF modules are recent (700 -> 2600 MHz for instance). Which means that many cash-strapped operators not having the capability to front the modernization CAPEX actually kept the entire 2G infrastructure running. This also means the basestation controllers (which are completely independent from the 3G radio network controllers, and independent from 4G's MMEs) also needed to be kept running. With hundreds/thousands of basestations and controllers running in your network, you're prone for hardware failures which will amass over time. As an operator you also cannot take risk in trouble resolutions/bug rectification/sw releases, so you typically contract your manufacturer to provide you yearly support for 2G. It's becoming at relaxed SLAs given that operators are refarming the 2G frequencies to 4G, but still in countries where the subscriber profile varies wildly (for example the MEA market: plenty of high-income users, and many more low-income users with cheap handsets that only support 2G/3G), operators cannot risk downtime on 2G because it will affect revenues. Of course this varies from country to country and operator to operator, with many now deploying 4G-only networks and benefiting from dedicated LTE bands without sharing with 2G/3G, but that's the general idea for the remaining existence of 2G and its related operating costs.