Cutting through all the theorizing and digression, here's the simple answer:
They are working on a new songbook that will have 150 songs total (just like the Psalms), will have updated lyrics that match the scripture quotations from the revised NWT, and will also have the same gray flexible cover. Instead of releasing it all at once like usual, they are doing it gradually, and by the time the new complete songbook comes out, it will have been about 7 years since the last one, not the shortest stretch between books that they've had.