As first it's only for the infotainment system. And the rear view camera is already a part it. And then at some point the manufacturer lets you adjust your adaptive cruise control through it. Then more and more critical things are integrated into what used to be "infotainment", until very soon the entire car can't run without the screen.

I think on most cars the infotainment systems runs on a separate computer that is connected to the drive computer via a bus. With a bit of commonsense design it should be fairly simple to have a car thats still useable without the infotainment computer.

It indeed is fairly simple, but the manufacturer has no incentives to do that.