There is no expression in your fingers, only expression would be how the sound reacts to your touch.
Organ depends on an expression pedal... only..
Hmmmm - first point: There can be MUCH expression from your fingers, even on a soft setting. Depends entirely on the touch. I like a little more resistance, but that isn't a universal choice.
Second point - While the expression pedal facilitates much greater control (in a more traditional way), the ability to play some notes louder, or softer than others can easily be done with touch response.