I've always thought that "Leaf by Niggle" was purely autobiographical in that Niggle is Tolkien himself. The 'leaf' of course would then be his universe of Middle Earth. Basically Tolkien was expressing the fear that he may be called to go on the journey i.e.die before his work was complete. Certainly there must have been a mountain of material left unedited at Tolkien's passing. Material that his son Christopher has been kept busy putting into publishable form for many years.