Stan Schroeder does the digital maths on Mashable:
But take notice of the percentages. Physical sales = down 13 percent; digital sales = up 34 percent. At this exact rate of growth, in five years the revenue from digital sales will be 12.5 billion. At the exact rate of decline, physical sales will be down to 7.9 billion in five years. Add the two numbers together, and you get around 20.5 billion in overall revenue. Currently, this number is 18.8 billion.
[Thanks to Michael M]