Imagine if, when Frodo and Sam have just fended off Gollum atop Mount Doom, suddenly, without any prompting or foreshadowing whatsoever, Eru Iluvatar appeared before them in all his divine glory, offering them a choice: to destroy the Ring (but also the elves and Gandalf, for... some reason), to replace Sauron as the new Lord of the Rings, or to instantly make everyone in Middle-earth part-orc or something. And Frodo, the same Frodo who has spent the entire trilogy doubting and arguing about the right thing, suddenly agrees with no prompting. Followed by a short chapter where Middle-earth is split apart with cracks in the ground, making travel impossible, the entire Fellowship is somehow stranded far away from Mordor and Frodo (including Sam, in a blatant continuity error), and nothing is really explained or resolved. The end.
Would most readers agree that it was within Tolkien's right to write such an ending, and criticizing him would damage his artistic integrity? Would it be right to say that people who bought the novel would only have issues with this ending because it's "bittersweet"? Or would it rather be because they saw that the work had unrealized potential, ruined at the last minute? And that's talking about a novel, a non-interactive medium where the reader has no control over the story.