GSM can easily share a frequency block with LTE/5G, so it's much easier to keep it around. 3G needs a whole 5 MHz frequency block for itself, but could you use

GSM can easily share a frequency block with LTE/5G, so it's much easier to keep it around. 3G needs a whole 5 MHz frequency block for itself, but could you use the same block and assign 4.5 MHz to LTE or 5G and just 0.5 MHz for a few GSM legacy devices. That's why it makes sense to turn off 3G, but keep GSM for older devices.