You are assuming that the driver's phone is the one connected. The typical use case is a couple, where either can drive. The first one to look up directions is the one who connects the phone. I do this all the time with my wife. Sometimes we have to disconnect one phone and connect the other so the passenger can continue using the phone without killing the map for the driver.
My kids usually manage the music, it's easy at the moment with an AUX lead so no real connection with the car. I can see as things are getting more connected this could have issues.
So glad right now that Apple took the bold step of removing the AUX plug. :)
Anecdote: I have only used CarPlay in rental cars, and every time I start driving while my wife connects her phone and gets directions and music going. It’s annoying that the map disappears when she checks her messages.
I use this all the time with my gf, I drive, she looks up a restaurant or something, opens maps to navigate to it and connects it to the car.
And after that she becomes annoyed she can't play Pokemon go or whatever game anymore :p
I usually have someone driving me, but would strongly prefer to control the music. With Carplay, if I connect my phone in order to control the music I'll lose navigation if I use my phone for anything at all.
Best solution I've found so far was putting my Spotify account on my drivers phone, then I can change the music using my phone without messing up waze.
On further reflection, I think it's still overall not a good idea. If the driver wants to change the music being played, or the volume, or whatever, and there is a passenger in the car with them, then I would think it would be okay for the driver to have the passenger do that on their behalf. And that might affect the display.
But I guess I can wait to see if iOS 13 can change my mind.