Sealing Kaguya isn't the solution.
When the Sage sealed Kaguya the first time around - he created Black Zetsu to ensure that she would return once more. This is why an allegedly wise man decided to distribute unnatural power amongst humans who were known for finding excuses to fight and kill each other. It was necessary for his sons to have something to fight over so that history could be washed away and re-written in the human mind. Then, the elder son's descendants would awaken Kaguya once again.
This is why he projected his essence into the future in the first place - to give his powers to Naruto and Sasuke and tell them each, independently, only what he wanted them to know (this is why Sasuke knows all about sealing Kaguya while Naruto was told nothing of the sort - The Sage knew that Kaguya would be revived).
Why?
That is a bit harder to explain at this point in time. It could be that he plans to make a return, himself - and plans to use Naruto and Sasuke to 'key' him in.
It could also be that he was unable to completely seal Kaguya and knew that the descendants of his sons could pull it off.
My bet is that his plan requires the attempt to seal Kaguya be made once more (perhaps there is a trap-seal of some kind that activates when the attempt is made to seal her again) and does not actually plan on it being successful.
In the end - the Sage has guided the entire process. The entire series is going to boil down to the fact that the Sage of Six Paths was no sage, at all, and was just a very powerful man who became entrapped in the affairs of the world and could not accept that people have the freedom to experience suffering.
