This decision has externals consultants written all over it: good idea on paper, terrible idea to implement. Even Tesla's own OS is terrible: the SatNav has no proper turn by turn; the music services cut out in tunnels, the integration with Messages is non-existent, etc.
Having said this, I am not keen on having the Apple aesthetic clash with the car interior, so if the app providers, including Apple apps, provide good APIs so that the apps integrate well on the GM OS, then this could be a way forward.
In the meantime, I love how Apple Maps fetches an address for an appointment from Calendar, sing my Contact list, and my Messages are all in one thread on my phone. (I share a car with my wife, so CarPlay also allows us to keep our Contact Lists, Messages, Music Playlists, etc separate)