The Sage said himself that somebody was going to surpass him to find "true power". Meaning the Sage thinks he failed in that aspect.
And again, what is with the "he beat the Juubi" bullshit.
He SEALED it. He removed the immediate threat, yes. But he did not DEFEAT it. As far as I know, parts of the Juubi are still around and ABLE to reform into the Juubi. It's kind of like a really delayed regeneration.
For all we know the Juubi was watching reruns of The Fresh Prince and the Sage snuck up on him and sealed him. A battle was only implied by the picture, but not known to be totally true.
On his deathbed, he created the moon and sealed the Juubi. Tobi implied that he used the Juubi's chakra to do so, meaning that he called forth its power to its own demise.
Is he a God in the Narutoworld? Currently. Will he remain the prime entity? Come on, you dumbasses think that the title character won't surpass him? You're naive and are probably way too into this anime.