The corollary of which is that no function the driver might sensibly need while underway should be presented or accessed through a touch screen.

So physical levers and buttons. With some tactile feedback, such as raised lips or centre bumps or distinct shapes, so without looking the user knows which one their finger is on (third from left in the Aircon group). Displays can be present multiple items (don't need physical dials) but changing that display when driving comes back to buttons.

Which is the problem, yes. It was perfectly fine to use physical buttons while driving because you don't have to look at them.

The law in the UK appears to be that is is perfectly OK to use a touch screen so long as you "stay in full control" and aren't holding the device:

https://www.gov.uk/using-mobile-phones-when-driving-the-law

NB My car has loads of features that I would expect to use while driving that can only be accessed through the touch screen.

If that's correct, why is it there at all?

(increasingly everyone including the safety wonks are realizing why this is a bad idea)