You call LOTR “the main story,” but if you had asked Tolkien, he would have said the SIMARILLION was his main story, his life’s work. Yet he was never able to complete it during his lifetime. Not because he did not care, however…
…Just for the sake of argument, let me point out that many many people invest their time into works without endings. F. Scott Fitzgerald never finished THE LAST TYCOON, Charles Dickens never finished EDWIN DROOD, Mervyn Peake never finished TITUS ALONE, yet those works are still read.
I do intend to finish A SONG OF ICE AND FIRE, of course… but doubtless Peake, Dickens, Fitzgerald, and Tolkien would have said the same.