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.

When you have that many layers, it takes a while to pop everything off the stack.

We went to see ROTK 12 hours before my wife was heading to the hospital to give birth. Her poor bladder almost didn't survive the numerous "is it over now?" endings.