The author never denied that publishers were very interested in making money in the past. The author’s point is just that their desire for money has increased.

This provides an alternate explanation for the decline in publication of books of intrinsic merits. If there’s been a decline in the quality of books written, that could reasonably mean there are fewer books of intrinsic merit available for publication.

If the profits were “unexpected,” the publishers didn’t know which books of intrinsic merit would lead to profits. So, we can’t read (C) as suggesting that the past publication of books of intrinsic merit was due to a desire to make profit.

This answer doesn’t suggest any reason to think there’s been an increase in the number of authors unwilling to publish. So, this doesn’t explain a decrease in the number of books of intrinsic merit being published.

This doesn’t provide an alternate explanation for a decline in the publication of books of intrinsic merit. If anything, it could support the author’s theory by suggesting that declining profits might have lead publishers to become more competitve and more concerned about money.