CarPlay doesn't run on the manufacturer's hardware. The car is just streaming a video stream from the phone, and sending back commands via touch screen, and control input. The manufacturers don't have to keep up with anything once the system is CarPlay compatible, the apps and everything are running on the phone. As long as Apple's updates are always compliant w/ the original CarPlay spec, then future OS updates won't affect the ability to use CarPlay.
CarPlay isn't replacing the built-in infotainment system. These cars will have their own menus, options, features, UI. CarPlay is just a feature that will let you play the Apple UI on the screen, and control the Apple software using touches, etc.
Not sure if you've ever used AirPlay on AppleTV, but it will work very similar to that, just optimized for use in-vehicle (large buttons, etc).
A lot of aftermarket head units offer "App Radio" for running apps from mobile devices and displaying output on the screen. CarPlay is basically equivalent to this.