Rosenthal, of Foner Books, is making this estimate based on an extrapolation from the Kindle sales rank graph. He goes on to say:
Kindle is a runaway train heading for Manhattan, and as it roars through Penn Station and under the city, it is shaking the foundations of New York’s oldest trade publishers. Increasing eBook sales mean increasing cost per unit for print books, since fixed production costs will be amortized over fewer print unit sales.
More details in the article.