Francis Ford Coppola and George Lucas both suffer from the same problem.
THEY DON'T KNOW HOW TO QUIT WHILE THEY ARE AHEAD!
They create a series of masterpieces... but they have to go back to that same series and modify them, and add to them, for the worse.
The Godfather and The Godfather Part II are two halves of one fully-functional organism.
The Godfather Part III was like diagnosing that organism with HIV or cancer.
Many are on-the-fence concerning Return of the Jedi. But it should have stopped there. No more.