I'm repeating what I've read someone say before, because I completely agree with it....
You really think that the person who raised the bijuu, whom the bijuu loved as a father, and whom all the bijuu declared that Naruto was a reincarnation of (or atleast reminded them of him,) will all of a sudden become the final villain????? Cmon.
Dumb and doubtful.
This scenario would also imply that once again, the Bijuu will be betrayed/used/become sad victims AGAIN... and I'm pretty sure that scenario has been overly played out already.