People aren't talking about buying fanfic the way you'd by a book. They're talking about custom ordering the exact fic you want about the exact characters you want to read about.

Free-to-read fanfic would still exist. Hell, chances are you'd be able to read the fanfic other people comission, because people in fandom like to share their squee. You just wouldn't be able to comission your very own custom ordered fic without ponying up a few bucks.