It may be inevitable, but I don't think it's anything new. There were huge masses of follow-up material to THE ILIAD, ODYSSEY and AENEID, nowhere near as good, of course. Everything from DON QUIXOTE to DIVINE COMEDY had vast piles of unauthorized sequels, follow-ups, pastiches and spoofs. I guess the concept of copyright and trademark soon wore that down. Although Edgar Rice Burroughs' heirs still have to occasionally crack down on someone trying to publish new Tarzan or John Carter stories without permission.
Certainly, there were cases where the later stories were in fact better than the originals by the creator. I can't think of many off the top of my head, though.