Why would you think so? Aftermarket CarPlay console coming this fall, costs between $500 and $700 | Ars Technica
because Because everything runs on the phone for CarPlay. It's just being mirrored to an external display and receiving some external input from touchscreen or buttons.
Paying $700 (for the low end Pioneer head unit) may still prove cheaper than the price Toyota will charge. The 2015 models should have CarPlay, but will it be an option, like Entune, or standard on the base model? The low end Pioneer unit lacks nav, but I assume the iPhone's Maps app would still be accessible (although I have seen no guarantee of that). For those not upgrading their Prius in the coming years, the Pioneer (and Alpine, and possibly other) units may be the best solution; although another thread on PriusChat notes a problem using the right side steering wheel controls with a third party unit (but that could be corrected). Getting the new head unit will also require instillation costs.
It would be great to have just a large display in the space where all of the buttons and the existing screen reside, then control everything on the screen from apps. I'd swap that out in an instant.
What on earth gave you this impression? In reality it will be very limited on what it will be allowed to run and display. It's a nav for a car, don't expect to be able to watch videos, browse the web, and play candy crush on it while you're driving.
The Pioneer head unit that has the compatable interface for the iPhone, is not what it seems, I tried my 4S and only some basic icons are mirrored and not all apps work in the mirror interface. It was said somewhere that a iPad would be better.
$500 for less features on the car when an Ipad works better and you can take it with you , and doubles as a tablet.. i'm fine with that...