The fact Go Set a Watchman was written FIRST then it spawned To Kill a Mockingbird yet it's being called a sequel shows our current culture.
I'm not sure I see that as a particularly modern problem. You've got the release vs. chronology ordering for series like the Narnia books (Magician's Nephew will always be a sequel to me, but that's not how many readers have seen it) or for authors whose later works were so successful that their earlier unpublished works were re-evaluated (Jane Austen.) - Jennifer Dittrich