What Ridukou said, is that there will come a time, when they (the 9 beasts) will be together again. In other words you could say, they will turn into the Juubi. We are seeing the Juubi right now, so there may be no reason to think deeper about it. Ridukou might even have meant this situation, since after humanity learned about ninjutsu, they could actually develop the power and tools to once more defeat the Juubi, although that would require them to be united, which was exactly Ridukou's goal.
Perhaps his plan all along, was to create a "sacrificial" clan with cursed eyes. Have them revive the Juubi, to force the rest of the world to unify, defeat it, and create peace for all. In the end, there was never really any other possible way, than something like this, to create a united front among all of humanity. In other words a union, which could last after the war.
Just like the world wars brought us FN and EU, and several other constellations. This is exactly the same. For there to become peace, there must be war. If what you state (though has been stated several times) happens, I believe, he would say exactly this to Naruto. What will the result be? Naruto must kill Sasuke to end the circle, or peace will never occur. U_U
