OTOH AT&T in the US has already shut down its 2G network and T-Mobile US has reduced their 2G capacity to the point where they can operate it in the guard bands

OTOH AT&T in the US has already shut down its 2G network and T-Mobile US has reduced their 2G capacity to the point where they can operate it in the guard bands for their 3G signal. 2G is basically on life support in the US at this point. The writing is also clearly on the wall for 3G as well, though the carriers won't be able to shut that down until enough people upgrade to VoLTE-capable handsets (even T-Mobile, who have the greatest VoLTE penetration among the US carriers, still see a third of their customers' calls go over their 2G and 3G networks).