I think you need buttons and the screen.
I can just use the volume knob on the steering wheel with my thumb to control volume or mute music, but if I'm parked and want to listen to a specific song it's fine that I go poking around on the touchscreen to do that.
I can use the 'mode' button to switch to the radio or Spotify, but if I want to set up Spotify with my account details I need to use the touchscreen.
The touchscreen lets you easily expose rarely used, complicated functions. Things you need to do while driving need to be buttons.