I half think this is by design and half think it's just been a cost saving measure up until this point. As much as I don't care, people like having their (always shitty in these implementations) aGPS as part of their infotainment units, as well as the various readouts. Cutting those features is mainstream market suicide.
That said, (note, I know very little about information systems), were I to be in charge of designing a car I think I'd have the foresight to see this possibility as being inevitable and would lean towards a closed system for safety critical systems (drive train!) and a connected infotainment system - with no actual physical connection between the two. Of course, you're going to need the infotainment system to have access to some of the safety critical systems for monitoring (because that's where you'd control HVAC) but I'd imagine that you could have redundant connections for these purposes rather than unrestricted access. It would cost more to design such a system and implement it, but it would limit your liability dramatically.
I DON'T KNOW WHAT THE FUCK I'M TALKING ABOUT.
On the nose. Totally separate systems would cost more, so they won't do them without some headline-grabbing tragedies. There is no reason for the ignition, drivetrain, safety and stability systems to be together with comfort electronics. Nothing should be able to manipulate your engine or brakes systems remotely--ever. I wouldn't even want the capability to query them for data readouts remotely, because you know, someone always seems to figure out buffer overruns or ways to crash systems.
This all reminds me of the movie Transcendence, where the military ended up resorting to old vehicles with absolutely no electronics, to combat the sentient AI which invaded all systems everywhere
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