You have correctly observed that the number of universes that got to experience the joy of a Hitch-Hiker's 4 in which Gimblethrop Tisslwheep put together a crac

You have correctly observed that the number of universes that got to experience the joy of a Hitch-Hiker's 4 in which Gimblethrop Tisslwheep put together a crack team of thieves for a heist in the glamorous, newly-built casinos of Squornshellus Zeta, never mind the stunningly thoughtful portrait of Eccentria Gallumbits's later years as a titan of the adult entertainment industry and the incisive, embittered critique of the continuing tragic effects of the British Empire's many sins that made up Hitch-Hiker's 18, is vanishingly small. But I will note that the idea of an SF/F author having a world to which they return again and again, with a mix of recurring characters and new ones, is pretty common. Asimov had the Foundation and the Robots books, and eventually tied them together. Heinlein had his Future History. Alan Dean Foster's Humanx Commonwealth absorbed a lot of my pocket money in the eighties. So did Larry Niven's Known Space. Tolkien's Middle-Earth. A new book by a beloved author in another corner of the same setting that their big success came from is an easier sell to that "subset of fans who are interested in what Arthur Dent is doing" than a totally different book by the same author.