C.S. Lewis, IMHO, simply didn't have the worldbuilding skill that made Tolkien's Arda a fixture in the popular imagination. His works simply didn't have the attention to detail and mythology that Tolkien created; he didn't truly care about his world, but instead used it as an opportunity to illustrate parables. I would expect fantasy to be more of a trend than a lasting genre, and to be marketed at a younger audience. In this way id would resemble the slate of derivative Harry Potter ripoffs in the early to mid 2000's. To compensate for this, we might see science fiction becoming more prominent; the ATL equivalent of Dungeons and Dragons, if anything like it exists at all, will be set in the far future. Overall, we have fantasy as a children's genre; people who were fantasy fans in their younger years would 'grow up' to science fiction; the two genres share many elements in common.