Among other zombie novelists, Jules Verne published The Sphinx of the Ice Fields in 1897 and H. P. Lovecraft published At the Mountains of Madness in 1936, both sequels to Poe's Narrative of A. Gordon Pym.Going back even earlier and moving specifically out of the field of the novel, there were several attempts to continue or complete Coleridge's long poem Christabel. I'll have to see if I can't find details and names; I had a complete list at one time.
The problem is not so much that people want to continue to hear new stories out of familiar and much-loved scenarios, or universes, or with characters they can't let go—the problem isn't so much with that as that the writing in these sequels is pretty undead. It almost never reaches the level of quality that inspired the original love. With very few exceptions, the followers are not as great writers as the originals. Actually, if anyone could pull off following in Chandler's footsteps, it's Parker. I read "Poodle Springs" some time ago, and thought it was pretty good. Not as great as the master, but pretty good. A few of the early Spenser novels are among the best I've seen in the genre for awhile, so . . . .