"They" refers to the programs that exist (or more broadly anything based on "statistical likelihood or mashing up the familiar", which is the overwhelmingly dominant sort); "never will" refers to "never will write the next great novel or create the next great painting", not "never will exist."
I think the author might agree that they never will exist, but because of his actual point about what they will do to creative work and communities, not any physical limitations.
I think the author might agree that they never will exist, but because of his actual point about what they will do to creative work and communities, not any physical limitations.