It's not a question of having the rest of
The Lord of the Rings ready and then inserting the Old Forest bits. As William points out, most of the story was 'discovered' after this episode. Tolkien didn't reflect on the defeat of Sauron and suddenly feel the urge to add an extra chapter on some dancing hermit.
But other people have explained well why it works; and I for (another) one love Tom and Goldberry!