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Volkswagen reintroducing physical controls for vital functions | Autocar
(www.autocar.co.uk)
This is a most excellent place for technology news and articles.
Without actually knowing how much constructing the physical buttons cost, I would guess that the real savings are in process optimization - if all you have for the interface is a screen, then you don't need to have the interface design done before constructing the car - you can parallelize these tasks.
Insufficient as far as justifications go, but understandably lucrative.