The thing about your argument here is that it is quite valid. It would mean everything that you said...in that respect would it not make sense that kishi is taking this direction based upon the plot decisions he has already made?
Indeed he could probably defeat the Juubi...assuming that it is still a threat...I think the kages will take the Juubi down and the Sasuke will show up to try to defeat Madara but will lose and that is where the theory fits in...either Sasuke loses before madara goes down or he takes madara with...
So just a theory as to where the plot no jutsu is going:
*Disclaimer*
I am no particular fan boy of either side of the Sasuke/Naruto debate. So please take this thread with as little bias as possible.
So my theory is that at some point Sasuke will be mortally wounded (if by Naruto's hand so...