we need Ridley here to explain himself..
I interpret his words to mean that SF has gone the way of the Westerns and will not be seen that often in the future. "Science Fiction is dead". Neither of us seems to agree with that.
It appears to me that he is wrong intellectually as you have elucidated. I believe he is also wrong from a purely prosaic standpoint. In addition to it's value as art and cultural commentary, it will continue to be a major part of the entertainment milieu, including the bombs and the brilliant.