"Advances have fallen, generally, for everything except the biggest potential bestsellers. Given all the changes, both economic and technological, SF hasn't done too badly"

This quote by Alan Dean Foster is referring to the advances made in the publishing industry, particularly in the sci-fi genre. He is stating that advances have actually reduced for the majority of books, but the biggest prospective bestsellers are still receiving advances. He is also noting that regardless of the economic and technological modifications that have actually happened, science fiction has not been affected too severely. This recommends that the genre is still popular and that publishers are still going to buy it. Foster is likely attempting to encourage authors to continue writing science fiction, regardless of the changes in the industry, as there is still potential for success.