"It is unclear if Apple, car makers, or both are to blame for the missed 2024 target."Welcome to Tim Cook's Apple, a land of broken promises
What's Apple supposed to do? Put out the next generation of CarPlay without dealing with automobile manufacturers?
Maybe this is completely on the heads of Apple's engineers and thus "Tim Cook's Apple" but it's just as likely that it's entirely the fault of the automobile companies. CarPlay is not something Apple has full control over top to bottom. It has to integrate into existing and future hardware (car computer and the car itself), firmware, and software on the vehicles for multiple different manufacturers.
It's a little like Microsoft and Windows having to work with a lot of different hardware except that instead of having the hardware designed for Windows, Apple is dealing with hardware designed without CarPlay in mind. Most/all of the automobile manufacturers design the cars and their own infotainment hardware and software and CarPlay gets more or less tacked on. That's not to say there is zero consideration of CarPlay, but it's highly unlikely to be the most important consideration a company makes when designing a car.
I'm not saying Apple isn't at fault. I'm saying there are other issues to consider.