What if....
What if the Slayer is unable to beat the First Evil 'cause she's not powerful enough?
What if she's not powerful enough because the "Slayer Power" is currently divided between Buffy and Faith?
What if that's why the First Evil is making his move now? 'Cause Buffy and Faith co-existing has weakened the "Slayer Power" and he thinks he can make his move? The "Death of Buffy" being referred to by the Big Eye isn't the most recent one - it's the one that activated Faith -- and the First Evil just took a few seasons to get his act together?
So, in the season finale (which will feature Angel), Buffy "gives up" her Slayer power, passing it to Faith (who will be featured in the final 5 episodes, I think) so that she can defeat the First Evil.
So, the series ends with Buffy, alive but normal, no longer having to give of herself to save the world. It's sorta been her underlying desire for the past few years -- to not have the responsibility.
Buffy walks off into the sunset, to find her place in the world and leave the world of "Buffy the Vampire Slayer" behind.
Also, SMG has said that she'd be happy to do guest spots as "Buffy" in any future spin-off, assuming that she survives the finale (she didn't know at the time of the interview, I think).
-JB