This is an intriguing issue, Esty.
The concern over spoilers isn't part of traditional literary culture. It's part of pop culture--did it arise on the internet with the discussion of weekly TV series and movies? (Not to self, check out OED on "spoilers" and see if the word postdates Tolkien.) I could be wrong, but I doubt Tolkien himself would have been bothered by spoilers. After all, look how many times he rewrote some of his own stories, how many versions we have. He was interested in definitive versions.
Books, for old time readers, were meant to be savoured for more than simply the plot. It was probably considered quite vulgar to be interested only in whodunnit or whahappened. It was the 'quality' of writing, the interest in how the writer choose to present the story and characters, that was supposed to provide the entertainment. Generations of clever young fellows spent (misspent?) their youth translating Greek and Latin not simply as an aid to demonstrating their knowledge of that language but to show their skill as writers in English, their command of style, technique, rhetoric, "colour." (Well, this was the pedagogical purpose behind the imposition of those school assignments.
)
Novelty was not an especially important quality in literary merit. After all, much of the reading public already knew the plot of the ancient stories. So why did writers retell those stories? Because they saw new and different ways to tell them, new and different approaches, perspectives. So it was almost a situation where knowing the plot ahead of time was part of the reading process, being able to make comparisons and see new twists and turns, being able to appreciate how one was led up to the conclusion. So it didn't matter if readers guessed from the Prologue that Frodo survives: the interest was in his internal journey and how he came to survive.
Novelty really I suppose only became significant with . . . novels.