Well, it's not that hard since if you look at what some of the well established brands have going for themselves, its a lot of inbetween models. BMW is a great example of this, among many others.
Then there's the whole platform, powetrain and drivetrain side of things which will get shared among a range of models to some extent. So it's not all that crazy as it might seem to be. They aren't going back to the drawing board with every model.