I would imagine that the internet provides a major disincentive for publishers to invest in non-fiction. Certainly, whenever my children are asked to research a subject for homework, they are given a string of websites to look at ... and give me a very withering look when I suggest that we head down to the library and find a good book on the subject. So I reckon the authors have a point.
And I must admit that we don't buy very many non-fiction books ... although we do have a fair number that DH and I had as children, which are still going strong!