If a tablet which requires you to switch apps from GPS to music and back again every time you want the info/functions at your fingertips was really ideal in a car, we'd just be using the Windows XP desktop with no need for front-end software.
There's a reason why OEM navigation units have a "front-end" interface. It's unsafe to be flipping between applications when driving.
There's a lot that needs to be improved in the Car PC world - front-ends, mediocre hardware accessories that are difficult to install, mediocre software, mediocre Bluetooth handsfree support, etc. But it's a platform that allows for far more features and options than tablets ever will.
Show me a proper front-end, offline maps, the ability to manage BT handsfree phone calls and Sirius/XM functionality on a tablet and then we'll talk about a tablet replacing the traditional car PC.