Very well said, Mr Doctorow.
I can remember an interview George Lucas gave in 1983 right around the time of the release of Return of the Jedi when he said that even though he'd originally planned there to be nine Star Wars movies, that he was utterly burned out by making them and he had no plans to do any more. And I can remember that at the time I felt a bit disappointed, but I simply said to myself that if Lucas didn't have it in him to create more stories in that universe, then I would. I, and countless others. The universe seemed so deep and rich with possibility that I didn't for a second accept the idea that those characters and stories would end whenever Lucas decided he wouldn't or couldn't make any more. And I know for a fact that there are a great many filmmakers my age who got into this industry as a direct result of wanting to make more stories like Star Wars.
Even though the Discworld stories mostly take place on just one (flattish) planet rather than an entire galaxy, that multiverse is still just as rich with potential as the Star Wars one... more so, if anything, due to all the genre-crossing play that Pratchett did in his world. When I think of the slippery cobbles of Ankh-Morpork, the gleaming icy spire of Cori Celesti, the windy villages of the Ramtops, the ancient history of the Chalk, or the unexpected mysteries of Klatch, XXXX, and the Counterweight Continent, or when I visualize Vimes, Nanny Ogg, Vetinari, the Low King, our favorite Igor and his dog Thcrapth, and a certain blue-eyed skeletal manifestation who couldn't speak lower-case letters if he tried (AND WHY SHOULD HE?), I remember all their stories, and it's easy for me to imagine many, many more, involving all these locations and the heretofore unmentioned spaces in between, and populated by these timeless, iconic characters and all the wonderful as-yet-unknown people and creatures that live and die and haunt and Morris-dance among them... it's so easy to dream of more Discworld adventures, romances, terrors, and laffs.
But when I think about how those stories might be realized in any media, written down for others to savor and enjoy, I feel pretty bleak indeed. Since, in a way I haven't felt since Douglas Adams died, I believe to the bottom of my soul that nobody now lives who could do it. Certainly I could never do justice to Pratchett's writing. And I don't think anyone else could either.
So yeah, we can get new Star Wars stories until the cows come home, there have been hundreds of perfectly serviceable Sherlock Holmes adventures conceived and written in the 85 years since Arthur Conan Doyle died, and, if anything, the Conan and Cthulhu Mythos stories have only gotten deeper and richer since Howard and Lovecraft passed on.
But there won't be any more new Discworld stories worth the name, just as there won't be any more Hitchhiker's Guide stories worth the name. And that is a truly depressing loss for us.