I would probably never be able to back this up with any sort of textual evidence, however. This is all just opinion and speculation. It has exceeded my expectations. I think he says somehing to the effect that they had greater leeway nearer the beginning of Time, but as Time progresses they have fewer and fewer choices before them. Hence, even they don't know how Time will end.
Personal choice is still important to Tolkien.
It sort of goes back to Iluvatar's statement concerning Melkor's deeds redounding to Iluvatar's glory. All the choices made by individuals will be taken up and incorporated into the ever-changing theme. Hence, having the freedom to choose, the Valar can depart from the theme they know. Hence, each Man plays only a brief role in the unfolding theme and cannot really affect it much. This is all just opinion and speculation.