From the brief amount of information available so far, CarPlay doesn't seem to preclude the in-car system from supporting other mobile phone OS's. For instance

From the brief amount of information available so far, CarPlay doesn't seem to preclude the in-car system from supporting other mobile phone OS's. For instance it seems possible to equip a vehicle with, let's say, the Open Automotive Alliance's in-car Android system that can use Project Mode with Android phones and also implement CarPlay for when iPhones are present. After all, CarPlay appears to be dependent on the phone's OS for all displays and whatnot, with CarPlay just being the protocols to link the in-car system to iOS in the necessary manner. This is just speculation, we can't know for certain until some of the technical details emerge, but it seems unlikely that car manufacturers would limit their systems interoperability to one platform.