> If their tight experience is so superior, why do they restrict competition? Wouldn't they win anyways?

iOS today regularly updates with improvements across the full software stack, up to the Apple apps themselves. Sometimes these changes are major. In the world the justice department is asking for, big changes to —for example, Messages app— would have to be coordinated with every app developer in that category. Changes would takes much, much longer, and in many cases would have to be watered down.