This is a fascinating POD for the development of fiction in general. One possibility is that fantasy never splits off into its own genre, which was a reaction to the commercial success of Tolkien. Instead there continues to be a combined "weird fiction" genre incorporating everything with non-realistic premises. The absence of Tolkien could butterfly both fantasy and science fiction as distinct genres.
Another possibility is that an alternate author replaces Tolkien by filling the same niche. In particular, it is interesting to imagine a world where the works of E.R. Eddison form the basis of post-war high fantasy. Eddison and Tolkien had many things in common, but their underlying moral values and aesthetic priorities were very different. Eddison was much less interested in mythmaking and conlanging, and he didn't subscribe to Tolkien's ultra-vanilla moral sensibilities. If the genre of fantasy had taken Eddison as its prototype, it would be more transgressive and perhaps more interesting as literature. With less of a focus on world-building as an artistic end in itself, authors might invest more time in characterization and narrative.
Another possibility is that an alternate author replaces Tolkien by filling the same niche. In particular, it is interesting to imagine a world where the works of E.R. Eddison form the basis of post-war high fantasy. Eddison and Tolkien had many things in common, but their underlying moral values and aesthetic priorities were very different. Eddison was much less interested in mythmaking and conlanging, and he didn't subscribe to Tolkien's ultra-vanilla moral sensibilities. If the genre of fantasy had taken Eddison as its prototype, it would be more transgressive and perhaps more interesting as literature. With less of a focus on world-building as an artistic end in itself, authors might invest more time in characterization and narrative.