Personally, I would even question "or their direct successors." But then again, my decision to ignore all Oz books after Baum's posthumous final opus, Glinda of Oz, is driven by a perceived nosedive in the quality of the writing, from that book (superb) to the only Ruth Plumly Thompson Oz book I ever read (definitely not superb).
I was thinking more in terms of a series like Star Trek or Star Wars which changes hands over time. Naturally there are no absolutes. "Canon" is just a shorthand label for something far more complicated and mutable than five letters can adequately cover. People ascribe too much importance to labels. They expect them to be the final word about what something is, when really they're the crudest beginning of understanding.
I mentioned in another thread that I grew up on The Bobbsey Twins. Like such sister series, Nancy Drew, The Hardy Boys, Tom Swift, &c, it was created by Edward Stratemeyer, certainly the first book packager in the children's literature market (and arguably the first book packager, period), with the vast bulk of the individual novels written by uncredited ghostwriters like Howard Garis and Nancy Axelrad, under house pseudonyms ("Laura Lee Hope" for The Bobbsey Twins, "Franklin W. Dixon" for The Hardy Boys), and under close editorial supervision (does this, except for the writers going uncredited and the books being released under house pseudonyms, sound familiar to readers of tie-in books? It should!). In that case, it would be the editorial staff that is the "keeper of canon," just as the executive producer and production staff that have that role for a television series.
Yes. As I said, it's about continuity of authorship. That can mean a single author or a group of creators.