That's not that feasible: they use the consumer-area electronics a lot now to allow configuration of the more critical systems, and to read data from them.
It's not feasible to lock my front door, because my house was built with a non-stop conveyor belt running from the mailbox to the kitchen.
The entire point of this ask-slashdot is to identify cars that DON'T integrate entertainment systems and wireless access with the safety critical electronics. Cars that DON'T do the dumb&dangerous stuff you just listed.
Data flow *from* the primary systems *to* entertainment&wireless systems is marginally acceptable, if it's a physically enforced one-way data flow using optocouplers or something.
I seriously want each car manufacture to have one employee on staff, who's sole job is say "YOU'RE FIRED" every time any idiot engineer wants to permit ANY data flow from entertainment-or-wireless systems into safety-critical systems. I don't care how limited the APIs are, I don't caret how encrypted it is, I don't care how cryptographically-secure the certificates are. If there's data flow into critical safety systems, it's effectively certain that it's going to be vulnerable. You don't connect safety-critical systems to wireless input, period.