This sounds like a good idea, until you realize that the head unit is typically the thing in the car with the most computing power (think AI workloads as well as a GPU and multiple ARM cores) and the thing in the car with the network connection.

A trivial use case which requires write access to the CAN bus is the navigation system informing cruise control of an upcoming hill.

Equally trivial would be the seat position memory or profiles being applied through the main touchscreen.

(I work for a company that is developing infotainment systmes)