OSX already has a touch interface, It's called a trackpad!
If the trackpad functionality of my Macbook was transferred to a touch screen, and the keyboard. likewise - as it is on the iPad, you would have an interface to OSX that is little different to what you have at the moment. A bit of finessing and cleverness would be required of the OS for sure, but to say all the OSX applications would need to be re-written is untrue. They aren't re-written for a Macbook.