I am hoping like hell this ends up being the case. Give me power, a place to put my own stuff and some details on the CAN bus and leave me to it.
I do not want to pay a premium for your slow, locked down, buggy / seldom-updated touch screen.
No, because they knew what they were getting into when they bought this truck. And I'm sure there will be a dozen DIY ways to add a more traditional sound system.
If passengers want to DJ, you can get one of those little FM transmitter thingies that plugs into a phone/table headphone port.
A lot of Bluetooth speakers today can fill a car with a sound wall better rear speakers used to. Apple says you just need two of their Bluetooth speakers to fill a room in a house with great stereo and reasonably good surround sound. The square footage of a car is generally smaller than the supported room size.
edit to add: if Slate is successful, I wouldn't be surprised if a decently sized ecosystem pops up around easily installed custom sound systems and the tablets (possibly with headphone jacks!) to control them.