Speaking of setting fire to the past versions of iOS and starting afresh, Stephen Hackett’s likening of iOS 7 to the 2002 iMac G4 is a brilliant example:
If you remember back to 2002 when Apple released the iMac G4, it was a huge departure from the G3-powered machines before it. However, looking at the whole timeline, it fits with the overall direction the product was heading in. The move to the LCD was a clear forerunner to the iMac G5, whose shape is still present in the iMacs of today.
When Tim Cook said that iOS 7 was the biggest change to iOS since the introduction of the iPhone, he wasn’t kidding. The UI is of course drastically different, but things like background-updating and better car integration are huge changes.