This might be a bit controversial...ok alot but.
We don't exactly know what the so6p has actually done other than teach people about the ninja way blah blah, he had the ability to seal the juubi away yea that itself is a big feat but, what if that was an ability just particular to him and he wasn't exactly special in everything else only compared to everyone else at that time he was? So, his power and ability could possibily be witnessed by edo madara at this moment, even if it is a taste, I may even go as far as saying edo madara might even be very close to the so6p's fullness half way there even maybe more.
There must be a limit to his power, that's for sure and his chakra is finite, he obviously couldn't rid the world of the juubi so he did the next best thing, sealed it and split it. That to me goes to show he wasn't able to fight it for an extended period of time?
I personally think the sage is overrated, like itachi says, everything has a weakness, the so6p can bleed therefore he must have a weakness, I mean if he was the best at everything...then why couldn't he use a superior form of oonkis dust technique that kept him alive for soo long, I can't recall the sage being famous for a long life.
Just my thoughts.
You may now flame me.
Edit: Just remembered he split the juubi near his death because he didn't want it to roam free after his death, so the part where i said he cant fight it for an extended period is irrelevant.
The latter paragraph however I feel is still relevant.