He says that about the commercialisation which is itself an effect of the movies.

In a way perhaps what he dislikes or regrets is that the movies have now become more genreally known than the books.

Especially if he don't think of the moveis as good representations of the story.

We should be careful not to put our own interpretations of the words as fact. What he says as I understand it is not that the movies will harm the books or make them dissapear, but that he doesn't consider the movies to be very accurate representations of the books. Either way the comments is about the movies.