As a long-time user of CarPlay, I used to think like the comments here. But having seen Rivian's user interface and the deep integration with the car that they have already implemented, and with their AI assistant just coming online, there is zero chance I would use CarPlay if I could use it. I can stream Apple Music, Tidal, Spotify (cough cough), and YouTube on the Rivian R2 I'm in line to buy.
Considering Apple was a complete failure on making a car of their own, there is simply no way I would expect their taking over the user interface of my car is going to lead to anything but mediocrity, and a complete mess. I don't need more computers in my car than necessary, and adding the iPhone for what will be useful is there in their Connect Plus subscription.
Do you think the navigation system of CarPlay is going to be able to plan my trip with details like where all possible charging spots are, how many slots at the SuperCharger are open, how much change I'll have when I arrive, how much charge I'll have at the end of the day, and which ones can do the best fast charge, tell me how long I'll be spending there charging, and what food places are nearby? Yeah, no, it won't give me all those details.
Why would Rivian hobble themselves using Apple's APIs? Y'all go buy the cars that don't care enough about owning the whole stack. I'll think different. (What other company does that? Hmmm...)