Joe Rosensteel, writing at his Unauthoritative Pronouncements blog:
I have no plan to purchase a GM vehicle, but I do rent cars. GM makes up a sizable portion of rental car fleets. At some point in the future those cars will no longer support CarPlay. I’m not going to sign up for a GM federated ID that stores my login credentials in their cloud. I’m not going to individually sign into apps in the car like Google Maps with my Google ID that I use for way more than just navigation. There’s no chain of trust with me and this random car from GM. No convenience that is achieved in exchange for increased exposure risk for storing my sensitive data in a car I don’t own.
The whole thing is good, but this point about rental cars (which I noted earlier briefly) is an important one: I won't take a GM rental car that can't use CarPlay.