The difference with the Apple knob is that you would be given a strong cue for what to do after you tap on the knob. The current knob design has no cue until you press and slide, when you see a value change. But you have to do it right before you get any cue. Real knobs are turned; not tapped, pressed or slid. The slider shows you an affordance that works. The knob shows you an affordance that does not.