I started the White Council almost two years ago hoping for the kind of discussions we have here. It has exceeded my expectations. This is a toughie. 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. Elves still have freedom of choice, but their nature seems to predispose them toward making certain types of choices more readily than certain other choices.