Thereâ(TM)s a pretty straightforward approach to this: Only implement the basic functionality like AC, blinkers, etc., implement it as bare-bones, secure and resilient as possible. Then rely on drivers bringing their personal devices (mobile phones and whatever will come after them) to interface with the car to provide navigation, entertainment and so on. Assistance systems (from lane keeping to autonomous driving) implemented as isolated, autonomous units not exposed to the network as to not be remotely hackable. As assistance systems evolve, those can be replaced (also by third-party ones).
Of course this requires clean architecture and clearly defined standards and interfaces. But itâ(TM)s a lot easier to create and maintain standards and interfaces than maintaining dozens of car operating system kitchen sinks. Heck, for their unicorns they could even provide the mobile applications if they want to fully control the experience (which they probably want), and standards, interfaces and phone apps would *still* be orders of magnitude easier to maintain than whole car OS.