It's 15 years since apples introduced us to a small but powerful piece of technology. That one device put apples growth through the roof. It became the first company to be worth a trillion dollars. And then the first to be worth two trillion and then three. The iphone is still bringing in the cash, it accounted for more than forty billion dollars in revenue in the last quarter alone. But what happens when at last everyone who's going to get an iphone has one?