You need more than "the standard audio protocol". The infotainment system should know a lot about the car's state (e.g., RPM, turn signal activated/deactivated).
The car can feed media and mixing controls to the amplifier; there's no need to send that elsewhere. Strict separation of concerns. You want to play a "door is a jar" announcement? Tell the amplifier to reduce all the other channels to one-third their current volume and raise the announcement channel, then undo that. No need to involve the phone or tablet or mini-PC or MP3 player or subether radio.
How would the navigation app on CarPlay know whether or not you have turned on your turn signal ahead of a turn? Or know to pause navigation because the car is no longer in gear?
People don't use turn signals in Massachusetts; that's considered giving information to the enemy.
The navigation app has a clock, GPS location, and acceleration and orientation sensors available to it in the phone. If you miss your turn, it will notice and find the next best route.
Why would you want to pause navigation on a gear change? Velocity plus the connection to the vehicle tells you everything you need to know. Zero velocity but still connected? Keep displaying stuff; you might be parking and you might be in traffic. Disconnected but velocity is high? The passenger is going to use you; keep navigation alive in the background. Disconnected and velocity is zero or walking? Go back to powersave mode.
I want the nav system to quit reminding me to turn when it notices that I have signaled. I want to pause navigation when I stop at the gas station or make some other stop (when not in gear, not gear change; the nav system should know when the vehicle has parked). Integration between the nav system and the rest of the car is table stakes at this point. It’s okay that you only want music. Some of us have higher expectations from our cars.
For example, the nav system should know when the battery charge or gas tank level is not enough to get to the destination.
I want the car to provide every bit of information it has to the phone so it can do smart things with it.
I rent a lot of cars in lots of places and I am amused at how poorly some manufacturers are handling this. Yes, treating the phone like an iPod works. That worked more than a decade ago.
(I was amused by your comment about Massachusetts. I am fully aware that Massholes have no respect for each other or the rule of law. Every time I return a rental car to Logan in one piece is a victory.)
The sad thing, is that somewhere out there there’s an infotainment PM at an automotive company taking note of every item in your list and saying “let’s implement stuff like that in our infotainment system! It’ll be a (jazz hands) differentiator!”
The auto manufacturers still have this delusion that they’re competing with Apple and Google on infotainment, instead of seeing the truth that better phone integration is not only the technically better solution, it’s something the customers want, and something that saves the company money. But you can’t get someone to understand something when their salary requires that they not understand it.
The car can feed media and mixing controls to the amplifier; there's no need to send that elsewhere. Strict separation of concerns. You want to play a "door is a jar" announcement? Tell the amplifier to reduce all the other channels to one-third their current volume and raise the announcement channel, then undo that. No need to involve the phone or tablet or mini-PC or MP3 player or subether radio.