There is also significant safety issues with basing separation by controlling using GS. With IAS the pilot can immediately respond with 'negative' or 'unable' when it's below or above a safe speed for normal flight, with GS you might turn a corner and the wind change might put your IAS outside of a safe region to comply. In a busy environment that would lead to extra radio and compensation vectoring. With IAS all planes are 'roughly' doing the same thing, when they turn a corner they are all flying in the same wind packet so should not vary in GS significantly if the controller is doing their job well. Also no autopilot holds GS only IAS as far as I know, so it would cause issues there as well.