You're not wrong, but ROTK has already been criticized for having multiple endings in the sense that the post-Battle of the Morannon parts drag on forever, and audiences were satisfied with the hobbits getting a happy ending without having to add an extended coda about Saruman calling himself "Sharkey" and the threat of sovietization coming to the Shire. It wouldn't have flowed well as a movie.

If ROTK had been made into a two-parter finale it could have worked, perhaps. Or maybe a separate spin-off film for the Scouring.