I was thinking more in terms of what made sense on Buffy's side. If the story wanted to say that she's done her work with the inner dark, she moves on from the vampires. Xander is the obvious human. Also, he'd represent a move from her need to look up to someone Angel, to more of a partnership thing. (Yes, Spike was also a more egalitarian thing -- but Buffy wasn't ready for that, or if she was it was only at the end of season 7).

The main reason I think a B/X end game is still possible, is that they've spent a lot of time on B/X this season. But I think it's more likely that there'll be a massive falling out after Dawn buys it (assuming she does)... and that enmity will be the big plot of season 9.