>One of them owned the system and drove it every day it still didn't seem like either one really understood it
Is it possible your friends are dumb? Because my 79 year old mother doesn't have an issue with CarPlay. The only potentially confusing thing is that it becomes your phone screen, and things you do on the phone influence what's on the CP screen.
The possible confusing-one-time scenario would be, say, driver using AppleMaps to navigate somewhere, and passenger using phone to look up the capital of Nebraska or whatever. When passenger pushes the home button to go to the browser, CarPlay will exit navigation and show ITS home screen, too.
Nav is still happening; you just have to switch back to it, just as you would on the phone.
And, again, this should really only be surprising once.
>Nobody riding in my (ancient) car has ever had trouble finding the big red/blue knob to change the temperature, or understanding "Here's my 2006 iPod if you want music, or unplug it and plug in your phone if you want". This is a solved problem.
CarPlay makes no effort to provide anything other than phone integration, so your climate control knob is safe.
Throwing shade on CarPlay by saying the problem was solved with an aux cable is pretty laughable, honestly. Sure, you could fiddle with your phone and play music, but there's no access to nav, no access to voice controls, and you have to grabble with your phone to change anything. CarPlay provides a large screen in a static location that simplifies the music approach, plus gives you access to other functions.
This is materially better than an aux cable.