"Safety critical" isn't just the drivetrain. I don't work in automotive and won't pretend to understand all the rules, but off the top of my head, some things that my car uses the head unit for:

* Backup Camera

* Turning traction control on/off

* Turning auto hold (maintaining the brake pedal while stopped) on/off

* Window defrosting

Many cars are even more integrated - are there any physical buttons inside a Tesla or is it all through the touchscreen?